Comprehensible Input
Comprehensible input is language a learner can understand despite not knowing every word — typically described as input one step beyond the learner's current level (i + 1) — and it is the central condition for acquiring a second language according to Stephen Krashen's input hypothesis.
The term was introduced by linguist Stephen Krashen in the early 1980s as part of his input hypothesis. The core claim is simple: we acquire language in one way only — by understanding messages. Grammar drills and memorization may produce conscious "learning", but fluent, automatic use of a language comes from large amounts of input we can actually follow.
Krashen captured the right difficulty level with the formula i + 1, where i is the learner's current competence. Input that is entirely understood (i + 0) teaches little that is new; input that is far too hard (i + 5) is just noise. The sweet spot is material you understand roughly 80–90% of, using context, images, and prior knowledge to bridge the rest.
In practice this means watching, listening to, and reading things that are interesting and slightly challenging: graded readers, slow-spoken video, dialogues with visual context, and stories. Because the brain is focused on meaning rather than form, vocabulary and grammar are absorbed implicitly — the same way children acquire their first language.
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FAQ
What is comprehensible input in simple terms?
It is language you can understand even though you do not know every word — for example a video or story that is slightly above your level but still makes sense thanks to context. Understanding the message is what drives acquisition.
What does i + 1 mean?
i is your current level of the language and +1 is the next small step up. i + 1 input is material just beyond what you already know — challenging enough to teach something new, easy enough to still follow.
Who came up with comprehensible input?
The linguist Stephen Krashen introduced it in the early 1980s as part of his input hypothesis of second-language acquisition.
How much should I understand for input to be comprehensible?
A common rule of thumb is roughly 80–90%. If you understand far less, the input is noise; if you understand everything, it is too easy to teach much.