i+1

Definition:

i+1 is the core operationalization of Stephen Krashen‘s Input Hypothesis: acquisition proceeds when learners are exposed to language input that is slightly beyond their current level of competence, where i represents the learner’s current interlanguage state and +1 represents the next acquisition target. Input that falls exactly at level i provides no new acquisition; input that is incomprehensible (i+7, for example) likewise drives no acquisition because it cannot be understood. Only input that is mostly comprehensible — but which contains some unknown forms embedded in an understood communicative context — triggers the acquisition mechanism. Though Krashen never formally quantified the threshold, later researchers and practitioners have converged on a rough guideline of approximately 90–95% known words per text as the operational zone for i+1 input. The concept is foundational to comprehensible input-based language teaching, the Dreaming Spanish methodology, AJATT, Refold, and nearly every input-first approach to language learning popularized online.

The i and +1

Krashen’s notation is deliberately simple:

i = current interlanguage. Whatever the learner currently knows and can process — not defined by a test score, a CEFR level, or a course curriculum, but by the actual implicit linguistic competence the learner has built up through prior acquisition.
+1 = one step beyond. The next structures, vocabulary items, or processing procedures that are developmentally ready to be acquired. Krashen draws on Pienemann’s Processability Theory as evidence that acquisition follows a fixed sequence — learners can only acquire what their current developmental stage makes them ready for. +1 is whatever falls in the next stage.

The key claim is that comprehension of the message — figured out via context, prior knowledge, nonverbal cues, repetition, images — is what drives the unknown form toward acquisition. The learner’s brain, having successfully recovered the meaning of the message despite the unknown element, maps the form onto its meaning and begins the acquisition process. Explicit analysis is not required; in fact, Krashen argues it is largely beside the point.

Why Not i+5 or i+10?

Input can be challenging without being useful for acquisition. If a learner encounters a text in which 70% of the vocabulary is unknown, the cognitive burden of decoding prevents comprehension of the communicative message, and therefore prevents the acquisition mechanism from firing. The +1 formulation specifies that the unknown elements must be contextually recoverable from what is already known. This is intuitively why learners report that native-speed authentic input in early stages is exhausting and unproductive — most of the unknown remains opaque — while carefully selected graded input (or extensively studied authentic input) is engaging and produces noticeable acquisition.

The 90–95% Threshold

Krashen never specified a precise vocabulary coverage percentage. The empirical precision came later:

Nation & Wang (1999) found that incidental vocabulary acquisition from reading requires approximately 95% known words per page.
Hu & Nation (2000) confirmed that text comprehension requires 95–100% coverage for adequate understanding.
Laufer (1989) proposed 95% as the lexical threshold for effective processing.

These benchmarks align with the subjective experience of graded reader design and with the frequency-coverage mathematics of real languages: at 95% known words, there is roughly 1 unknown word per 20 — enough to encounter new vocabulary in context without blocking comprehension. At 90%, there are 2 unknowns per 20 — still manageable for strong learners but stressful at lower levels.

The Refold movement operationalizes this via their “sentence mining” practice: learners search for “1T” sentences — sentences with exactly 1 unknown (1 Target) — for their Anki decks. This is a direct practical implementation of i+1: each mined sentence is in the learner’s +1 zone, understood except for one new element, making acquisition maximally efficient.

i+1 and Language Levels

The i+1 principle strongly influences how learners should select content:

At beginner stages: Authentic native-speaker content is far too advanced. Beginner learners need heavily graded input — children’s books, structured input courses, graded readers, or purpose-built platforms like Dreaming Spanish Beginner tier or structured beginner Japanese CI channels.

At intermediate stages: A broader range of authentic content becomes accessible. Learners at this stage can tolerate higher rates of unknown vocabulary because their background knowledge, grammatical sense, and context-inferencing skills compensate. This is the notorious “intermediate plateau” zone where learners often stall by leaving graded content prematurely.

At advanced stages: Nearly any authentic content is in the i+1 zone for at least some vocabulary or pragmatic features, and exposure to diverse authentic material drives continued refinement of the procedural system and register awareness.

History

1977–1981 — Krashen formulates the Input Hypothesis: The i+1 principle emerged from Krashen’s synthesis of natural order research, the Monitor Model, and acquisition-learning distinctions through a series of working papers and conference presentations. It was central to his 1982 book Principles and Practice in Second Language Acquisition.

1985 — The Input Hypothesis: Issues and Implications: Krashen devoted a book-length treatment specifically to the input hypothesis, elaborating the +1 principle with examples from classroom data, comprehension research, and L1 acquisition parallels (where parents naturally calibrate “motherese” to be i+1 for their children).

1980s–1990s — Nation’s vocabulary coverage research: Nation, Hu, Laufer and others empirically established the ~95% threshold that gave the intuitive i+1 principle a quantitative operationalization for vocabulary selection in reading.

2000s–present — Online dissemination: The AJATT community, Matt vs Japan, Dreaming Spanish, and Refold all built practical acquisition systems around the i+1 principle, bringing it from academic SLA into wide public awareness. The “1T sentence” concept in Refold is the most precise practical codification.

Common Misconceptions

“i+1 means studying grammar one level above your current level.”

Krashen explicitly rejected grammar-level operationalizations of i+1. The +1 refers to implicit acquisition targets in the interlanguage, which do not correspond to textbook grammar units. Completing “Chapter 6” before “Chapter 7” is a pedagogically organized progression, not an i+1 progression. Authentic comprehensible input, calibrated by comprehension rate rather than grammar syllabus, is what Krashen’s model requires.

“Krashen’s i+1 has been proven.”

The acquisition mechanism Krashen proposed is not directly testable with current methods because the interlanguage state i cannot be precisely measured independently of acquisition. The +1 principle is more of a heuristic that guides input selection than a falsifiable scientific claim in its original form. The vocabulary coverage research (Nation et al.) validates a quantitative operationalization, but this is not the same as validating the full acquisition mechanism Krashen hypothesized.

“i+1 means any input above your level is acquisitionally useless.”

This overextends the principle. Input slightly beyond the i+1 zone (i+2, i+3) is often still partially comprehensible and can produce acquisition, particularly for motivated adult learners with strong context-inferencing skills. The i+1 zone marks optimal efficiency, not a threshold below which all input fails.

Criticisms

Vagueness and non-falsifiability. Critics, including Gregg (1984) and McLaughlin (1987), have argued that the notion of “current level i” is too poorly specified to generate testable predictions. Without an independent measure of i, the hypothesis cannot be distinguished from other acquisition-driving explanations.

Output neglect. Merrill Swain‘s research on French immersion students in Canada demonstrated that comprehensible-input-rich environments did not produce nativelike production, suggesting that comprehensible output — the “output hypothesis” — was required in addition to input. i+1 specifies nothing about the role of production in acquisition.

Interactionist critique. Long’s Interaction Hypothesis argues that negotiation of meaning in conversation — clarification requests, recasts, confirmation checks — is what makes input comprehensible and drives attention to form in a way that purely receptive input does not. Krashen’s i+1 principle treats input as if comprehensibility is a fixed feature of a text, ignoring the interactive meaning-negotiation process.

Affective filtering. Krashen himself acknowledged this partially: even perfect i+1 input will not be acquired if the learner’s anxiety blocks it. But this is a post-hoc explanatory escape hatch — any failure to acquire from apparently appropriate input can be attributed to affective filtering, which threatens the falsifiability of the model.

Social Media Sentiment

i+1 has achieved unusual mainstream adoption in language learning communities. On r/languagelearning, r/LearnJapanese, and r/LearnKorean, “find i+1 content” and “mine 1T sentences” are standard beginner recommendations. The concept is so pervasive that many users apply it without knowing it has a name.

The Dreaming Spanish community is implicitly built on i+1 — the entire tier system (Comprehensible Beginner, Intermediate, Advanced+) is an implementation of calibrating input to the learner’s comprehension threshold. AJATT purists take a harder i+1 line, insisting on near-total-comprehension immersion material from the start, while Refold offers a more systematic operationalization via the 1T sentence mining framework.

Debates typically center on whether the 90% or 95% threshold is more appropriate, and whether learners can “push through” i+20 input efficiently enough.

Last updated: 2026-04

Practical Application

Assess comprehension rate honestly. Before choosing content, estimate how many words per page you don’t know. At 90%+ comprehension, the material is likely in your i+1 zone (especially with context support). Below 80%, reconsider — the cognitive overhead may exceed acquisition benefit.

Use frequency-ordered vocabulary resources. High-frequency vocabulary lists (top 2,000–5,000 words) represent the fastest path to i+1 access to authentic native content. Knowing 95% of words in typical authentic text requires roughly 3,000–5,000 lemmas, which is achievable with systematic spaced repetition.

1T sentence mining in practice: When reading or listening, extract sentences that have exactly one unknown element (word, grammar pattern, or usage nuance) for review. Add these to Anki with context. Over time, the volume of 1T sentences in any given text increases as your lexical coverage grows.

Graded content at early stages is not “cheating.” Purpose-built CI content like Dreaming Spanish Beginner or graded Japanese YouTube channels provides i+1 input for learners who do not yet have the vocabulary coverage to access authentic material. This is the fastest route to authentic input, not a detour from it.

Related Terms

Research

Krashen, S. D. (1982). Principles and Practice in Second Language Acquisition. Pergamon Press. [Summary: The source text for the i+1 principle — Krashen presents the input hypothesis with the i+1 formulation as the mechanism for acquisition, distinguishing it from both rote learning and comprehensible-output-only approaches.]
Krashen, S. D. (1985). The Input Hypothesis: Issues and Implications. Longman. [Summary: A book-length defense and extension of the input hypothesis, addressing critiques from Gregg, McLaughlin, and others, and elaborating the +1 principle with classroom and naturalistic evidence.]
Nation, I. S. P., & Wang, K. M. (1999). Graded readers and vocabulary. Reading in a Foreign Language, 12(2), 355–380. [Summary: Empirical vocabulary coverage research establishing that effective reading for acquisition requires approximately 95% known words per text — the quantitative operationalization of Krashen’s i+1 heuristic.]
Hu, M., & Nation, I. S. P. (2000). Unknown vocabulary density and reading comprehension. Reading in a Foreign Language, 13(1), 403–430. [Summary: Systematic study of the coverage percentage required for adequate reading comprehension — findings support the 95–100% range, validating the practical 95% threshold used in sentence mining frameworks.]
Laufer, B. (1989). What percentage of text-lexis is essential for comprehension? In C. Lauren & M. Nordman (Eds.), Special Language: From Humans to Thinking Machines (pp. 316–323). Multilingual Matters. [Summary: Early quantitative study proposing 95% vocabulary coverage as necessary for fluent reading comprehension — one of the first empirical attempts to operationalize i+1’s coverage prerequisite.]
Gregg, K. R. (1984). Krashen’s monitor and Occam’s razor. Applied Linguistics, 5(2), 79–100. [Summary: A major early critical response to Krashen’s model — argues that i, the acquisition mechanism, and the role of +1 input are ill-defined to the point of unfalsifiability. The most-cited methodological critique.]
Swain, M. (1985). Communicative competence: Some roles of comprehensible input and comprehensible output in its development. In S. Gass & C. Madden (Eds.), Input in Second Language Acquisition (pp. 235–253). Newbury House. [Summary: Swain’s foundational paper on the output hypothesis — cites immersion student data to argue that i+1 comprehensible input alone does not produce nativelike grammatical competence, necessitating the addition of output-based acquisition drivers.]
Long, M. H. (1983). Native speaker/non-native speaker conversation and the negotiation of comprehensible input. Applied Linguistics, 4(2), 126–141. [Summary: The foundational paper of the Interaction Hypothesis — argues that comprehensible input is primarily achieved through negotiation of meaning in conversation rather than through the properties of fixed texts, critically extending and partially challenging the i+1 principle.]
Pienemann, M. (1998). Language Processing and Second Language Development: Processability Theory. John Benjamins. [Summary: Processability Theory — the developmental stage framework that Krashen’s +1 implicitly references; provides the theoretical mechanism for why certain structures are acquirable at certain interlanguage levels and not before.]