i+1 (Comprehensible Input)
i+1 is Krashen's formula for optimal comprehensible input: language slightly beyond the learner's current level (i), where "i" represents current competence and "+1" is the next learnable element understood through context.
The i+1 formula is the operational heart of Krashen's Input Hypothesis. "i" represents a learner's current interlanguage — the sum of everything they have already acquired. "+1" denotes the next increment of language that is slightly beyond current competence but still understandable given context, prior knowledge, paralinguistic cues, and shared world knowledge.
Crucially, i+1 is not a syllabus item or a difficulty score — it is emergent. Krashen argued that if a learner is provided with sufficient comprehensible input, the next structure is acquired naturally and in the right order (connected to the Natural Order Hypothesis). You do not need to isolate and teach i+1 explicitly; the right input environment makes acquisition happen automatically.
In practice, i+1 means finding content where you understand most of the message but encounter some unfamiliar vocabulary or grammar. For example, a B1 learner watching a well-supported B1-B2 video with visual context gets continuous i+1 exposure. This is why CEFR-level filtering, context-rich videos, and visual/subtitle support are all serving the same underlying goal: maximising the proportion of comprehensible i+1 in the input stream.
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FAQ
How do I know if content is at i+1 for me?
A practical rule of thumb: you should understand roughly 95–98% of the words. If you need a dictionary for every sentence, it's beyond i+1. If you understand everything effortlessly, it's at or below "i". The sweet spot is content where you understand the overall message but occasionally encounter unfamiliar items you can infer from context.
Is i+1 the same as the "comprehension threshold" researchers measure?
Related but not identical. Researchers like Nation estimate 95% word coverage for incidental vocabulary acquisition, and 98% for comfortable extensive reading. Krashen's i+1 is a structural/grammatical concept, not purely lexical, but in practice the coverage thresholds serve the same purpose: maximising comprehensible exposure.
Can watching content that is too difficult ever help?
Not directly. Input at i+5 or i+10 — well above current competence — is not comprehensible, so it does not drive acquisition. However, visual context, captions, and prior knowledge can bring challenging content back to i+1 territory. This is why subtitles, scene context, and pre-teaching vocabulary dramatically expand what counts as comprehensible.