The Input Hypothesis (i+1) states that language learners acquire new language when they understand input slightly beyond their current proficiency level. In this formula, ‘i’ represents the student’s current ability, while ‘+1’ indicates the next developmental stage they can naturally acquire through comprehension rather than explicit instruction.
For ESL teachers, this means strategically calibrating your classroom language, materials, and activities to challenge students just enough to promote growth without causing frustration. This guide provides 8 practical implementation strategies you can use immediately, along with assessment techniques for identifying student levels and troubleshooting common calibration mistakes.
What Is the Input Hypothesis (i+1)?
The Input Hypothesis (i+1) is Stephen Krashen’s theory that language acquisition occurs when learners comprehend messages containing structures slightly more advanced than their current level. Developed in the 1970s-80s, this hypothesis challenges traditional grammar-focused instruction by positioning comprehensible input—not explicit teaching—as the primary mechanism for developing language competence.
The i+1 Formula Explained
In the formula i+1, ‘i’ represents a learner’s current interlanguage (existing knowledge), while ‘+1’ represents the next increment of language within their capacity to acquire. Krashen emphasizes that the ‘+1’ element is “roughly-tuned” rather than precisely calculated—natural, meaningful communication automatically provides appropriate challenge when learners engage with comprehensible content.
Key principle: Input must be comprehensible. Learners need to understand the overall message even if some individual words remain unfamiliar. Context, visuals, gestures, and prior knowledge make input comprehensible.
Important clarification: While language educators commonly reference 70-90% comprehension as optimal for acquisition, Krashen himself did not specify exact percentages in his original research. This guideline emerged from practitioner experience rather than Krashen’s theoretical framework.
How i+1 Differs from Traditional Grammar Teaching
The Input Hypothesis fundamentally differs from grammar-translation methods in several ways:
Traditional approach:
- Teaches grammar rules explicitly
- Follows predetermined grammatical syllabus
- Emphasizes error correction
- Requires immediate output practice
Input Hypothesis approach:
- Develops implicit grammar knowledge through comprehensible input
- Allows natural acquisition order
- Focuses on message comprehension
- Permits silent period before output
The Input Hypothesis is one component of Krashen’s five hypotheses that together form his Monitor Model, including the Acquisition-Learning distinction, Natural Order Hypothesis, Monitor Hypothesis, and Affective Filter Hypothesis.
How Do You Identify Students’ Current Language Level (i)?
You identify students’ ‘i’ level through three complementary methods: WIDA Can-Do Descriptors for formal proficiency assessment, ongoing formative checks during lessons, and observation of comprehension signals through student responses and body language. The goal is understanding what students can do with language now, not cataloging what they don’t know.
Using WIDA Can-Do Descriptors
The WIDA framework specifies what language learners can do at six proficiency levels (1-Entering through 6-Reaching) across listening, speaking, reading, and writing domains.
Quick reference examples:
- Level 2 (Emerging): Follow two-step oral directions; identify facts from illustrated text
- Level 4 (Expanding): Interpret information from complex sentences; produce text approaching grade level
- Level 6 (Reaching): Perform comparably to native English-speaking peers
Access WIDA Can-Do charts through your ESL specialist or online at WIDA’s public resources. These descriptors reveal not only current ability but also which supports (visual, graphic, interactive) benefit each proficiency level.
Quick Formative Assessment Techniques
Non-verbal comprehension checks provide immediate feedback without raising anxiety:
- Color cards: Green = understand, yellow = somewhat confused, red = lost
- Drawings: Ask students to illustrate what they heard
- Physical response: Act out instructions or point to correct images
- Traffic light highlighting: Students color-code text (green = known vocabulary, yellow = somewhat familiar, red = unknown)
Strategic questioning reveals comprehension depth:
- Emerging learners: Yes/no or either/or questions
- Developing students: Short answer questions requiring phrases
- Advanced learners: Open-ended questions demanding explanation
Managing Mixed-Ability Classrooms
Provide differentiated supports for the same core content rather than separate lessons:
| Proficiency Level | Support Strategy | Example |
|---|---|---|
| Beginning | Heavily illustrated + sentence frames | Picture-based story with “I see ___” frames |
| Intermediate | Visual support + vocabulary glosses | Same story with some illustrations + word bank |
| Advanced | Grade-level text + minimal scaffolding | Full text with challenging vocabulary only |
This approach ensures all students access the same themes and topics while receiving appropriately calibrated i+1 input for their level.
What Are the 8 Most Effective i+1 Implementation Strategies?
The eight core strategies combine visual support, scaffolding techniques, strategic speech modification, authentic materials, comprehension monitoring, story-based activities, sheltered instruction, and technology integration. Each strategy makes linguistic input comprehensible while naturally embedding structures slightly beyond current student ability.
Strategy 1: Visual Support Integration
Present visuals before or simultaneously with verbal input—not after:
- Realia: Real objects students can touch/manipulate
- Images: Photographs, illustrations, video clips
- Graphic organizers: Venn diagrams, flow charts, concept maps
- Gestures: Body language and facial expressions conveying meaning
Example: When discussing a story, display key scene images so students understand the narrative even when vocabulary exceeds their level. The visual context scaffolds i+1 input comprehensibility.
Strategy 2: Scaffolding with Sentence Frames
Provide grammatical structure while students supply content words:
- “I prefer ___ because ___”
- “My favorite ___ is ___”
- “The most important reason is ___”
These frames allow participation in complex discussions before students have fully acquired these structures—the essence of i+1.
Strategy 3: Modified Teacher Speech
Adapt your speech without oversimplifying content:
- Speak at moderate pace with clear articulation
- Use shorter sentences (10-15 words)
- Pause between idea units
- Employ strategic repetition and rephrasing
- Avoid idioms and complex phrasal verbs for beginners
Critical: Don’t limit vocabulary to only “taught” words. Natural speech with context provides better i+1 than artificial restriction.
Strategy 4: Authentic Material Selection with Scaffolding
Match authentic materials to proficiency plus appropriate support:
- Beginners: Simple children’s books, short animated videos, predictable songs
- Intermediate: Young adult literature, news articles on familiar topics, TV clips with subtitles
- Advanced: Unmodified adult content (podcasts, opinion articles, documentaries)
Before using authentic materials:
- Preview for potentially confusing elements
- Pre-teach 5-8 key vocabulary items with visuals
- Provide background knowledge about cultural references
- Chunk longer materials with comprehension checks
Strategy 5: Comprehension-Focused Questioning
Ask one question per minute of class time—vary complexity by student proficiency:
For emerging learners:
- “Is this a dog or a cat?”
- “Point to the red circle.”
For developing students:
- “What color is the dog?”
- “Where is the character going?”
For advanced learners:
- “Why do you think the character made that choice?”
- “How would you have handled this situation differently?”
This differentiated questioning ensures every student receives i+1-appropriate input during whole-class discussions.
Strategy 6: Story-Based Input Activities
Stories provide ideal i+1 conditions through predictable structure, emotional engagement, and natural repetition:
TPR (Total Physical Response) Storytelling:
- Tell simple stories using gestures students perform
- Example: “The character walks to the market [students walk in place], buys three apples [hold up three fingers], and returns home [walk back]”
- Physical movement makes language comprehensible
Class Co-Created Stories:
- Students suggest character, location, problem
- Teacher narrates incorporating suggestions
- Natural repetition: “The character went to the beach. At the beach, the character met a dolphin. The dolphin at the beach said…”
Strategy 7: Picture Talk and Movie Talk
Picture Talk—describe a detailed image using target language:
- Project interesting photograph or illustration
- Describe observable details with gestures and pointing
- Ask questions about the image
- Layer complexity gradually based on student responses
Early description: “There’s a dog. The dog is big.”
Intermediate: “The brown dog is running toward the beach.”
Advanced: “This enormous dog appears to be bounding enthusiastically toward the shoreline.”
Movie Talk—narrate short video clips (2-5 minutes):
- Select engaging content (animations, commercials, short films)
- Narrate while pausing frequently
- Control playback to repeat, rephrase, or check comprehension
- Moving visuals provide rich context for i+1 input
Strategy 8: Technology-Enhanced i+1 Input
Digital tools that support comprehensible input principles:
- Captioned videos: YouTube with same-language subtitles provides dual input channels
- Graded reader platforms: Newsela, ReadWorks adjust article complexity while maintaining content
- Interactive stories: Apps featuring audio + images + embedded comprehension questions
- Adaptive systems: AI-assisted platforms that adjust difficulty based on student performance
Caution: Many language apps emphasize translation and decontextualized drills rather than comprehensible input. Evaluate whether tools provide meaningful context and natural language use.
How Do You Know If Input Is Calibrated Correctly?
Your input is correctly calibrated when 70-90% of students demonstrate comprehension of the main message through responsive actions and accurate comprehension question answers, while still encountering some unfamiliar elements that challenge growth. Observable student behaviors provide immediate diagnostic feedback.
Positive Calibration Indicators
Students show these behaviors with appropriately calibrated i+1:
- Engaged facial expressions (leaning forward, nodding at key points)
- Asking about specific unfamiliar words: “What does ‘migration’ mean?”
- Successfully completing tasks based on input
- Requesting clarification on details, not expressing complete confusion
- Using context clues to figure out new vocabulary
Warning Signs Input Exceeds i+1
These behaviors indicate input is too difficult (i+2 or beyond):
- Confused or discouraged facial expressions
- Disengaging from activity or giving up quickly
- Asking how to spell/define multiple words per sentence
- Producing responses that completely miss the main idea
- Expressing frustration: “I don’t understand anything”
Immediate correction: Add more visual support, provide additional background knowledge, or simplify language complexity.
Common Calibration Mistakes to Avoid
Mistake 1: Confusing “comprehensible to teacher” with “comprehensible to learner”
Abstract concepts, idioms, phrasal verbs, and cultural references that seem obvious to native speakers often remain opaque to learners.
Mistake 2: Assuming uniform proficiency across all students
Students at the same proficiency band may have different strengths—one comprehends spoken language well but struggles with reading; another excels at reading but needs extra listening support.
Mistake 3: Oversimplifying to avoid challenge
Limiting vocabulary to only explicitly taught words provides i+0 (no challenge), keeping students at current proficiency indefinitely. Krashen noted caretaker language is “roughly-tuned” not “finely-tuned”—natural communication contains beneficial variability.
Mistake 4: Forcing output before readiness
Requiring speaking before sufficient input raises anxiety (affective filter) and may inhibit acquisition. Allow silent periods—output emerges naturally when students accumulate adequate comprehensible input.
How Long Before Students Should Speak?
Students should receive a “silent period” of no forced output—this period varies from weeks to months depending on individual learner readiness. Output (speaking/writing) emerges naturally after sufficient input accumulation and should never be coerced.
Involving Silent-Period Students
Effective participation methods that don’t require speech:
- Physical responses: Pointing, acting, arranging pictures in sequence
- Visual responses: Drawing, using yes/no cards, color-coding
- Written responses: One-word answers, sentence completion
- Digital responses: Clicking choices, dragging items, typing in chat
When Output Naturally Emerges
Students begin producing language when:
- They’ve received substantial comprehensible input
- They feel psychologically ready (low affective filter)
- They have something meaningful to communicate
- The environment feels supportive and low-stakes
Teacher role: Create opportunities for voluntary output, never forced performance. Accept approximations and focus on message over form during early production stages.
FAQ: Input Hypothesis Implementation
Q: Can students acquire language without explicit grammar instruction?
A: Yes—learners acquire grammatical competence through comprehensible input without explicit instruction. According to Krashen’s research, acquisition (subconscious development through understanding messages) differs from learning (conscious rule knowledge). While some metalinguistic grammar knowledge may help older learners edit writing or formal speech, the ability to use language fluently and spontaneously develops through comprehensible input, not studying rules.
Q: What’s the relationship between input and the affective filter?
A: The Affective Filter hypothesis explains why comprehensible input alone doesn’t guarantee acquisition. Negative emotions—anxiety, lack of confidence, low motivation—create a mental block preventing input from reaching the language acquisition system. Even perfect i+1 input cannot promote acquisition when the filter is “up.” Lower the filter through supportive classroom climate, reducing performance anxiety, providing interesting content, and never forcing output before readiness.
Q: Should teachers use students’ first language (L1) in class?
A: Strategic, limited L1 use can facilitate comprehension and lower anxiety, but comprehensible input must be in the target language for acquisition to occur. Research by Macaro (2005) suggests L1 serves best as a facilitator to increase comprehensible L2 input amount. Teachers might use L1 briefly for complex instructions, critical safety/academic vocabulary, or relationship building, then maximize time providing comprehensible target language input.
Q: How does this work with required grammar curriculum?
A: Provide comprehensible input where target structures appear naturally in meaningful context rather than through isolated drills. If curriculum requires past tense, create class stories about weekend activities where past tense verbs occur repeatedly in comprehensible context. Read stories where characters recount past events. Have students ask each other about previous experiences. The structure becomes input students comprehend while focusing on meaning—natural acquisition occurs through exposure rather than explicit rule-teaching.
Q: Is i+1 appropriate for all ages?
A: The core principle applies across age ranges, though implementation details vary. Young children naturally acquire through comprehensible input, while adolescents and adults also benefit substantially from input-rich instruction. However, older learners may benefit from limited explicit grammar instruction (particularly for writing/formal registers) that younger learners don’t need. Adults often experience higher affective filters due to self-consciousness about errors, making attention to emotional factors especially critical.
Q: How do I assess whether students have acquired target structures?
A: Observe spontaneous language use in meaning-focused contexts rather than testing isolated forms. After providing substantial i+1 input containing target structures, create authentic communication tasks where students naturally use the language: narrating experiences, discussing preferences, explaining processes. Accurate spontaneous use without conscious focus on form indicates acquisition has occurred. Ability to complete grammar worksheets indicates only conscious learned knowledge, not acquired competence.
Implementing i+1 Successfully in Your Classroom
Begin with one strategy from this guide—practice until natural before adding another. Many teachers start with Picture Talk or enhanced visual supports, gradually incorporating additional techniques as comfort grows.
Three essential implementation principles:
- Know your students’ ‘i’ levels through WIDA descriptors + formative assessment
- Provide comprehensible input at i+1 using the eight strategies outlined above
- Lower the affective filter through supportive environment and no forced output
Expected timeline for observable results:
While specific timelines vary widely based on factors like input amount, student age, and prior language learning experience, teachers commonly report increased student engagement within 2-3 weeks of implementing comprehensible input approaches, with measurable proficiency gains becoming evident over 2-3 months of consistent practice.
Most important insight: You don’t need to precisely calculate what structures each student is ready to acquire. Focus on providing interesting, comprehensible content through natural communication, appropriate scaffolding, and varied modalities. Trust the acquisition process—when students understand messages slightly beyond their current ability in supportive environments, they naturally develop language competence.
