i+1
Material at your current level (i), plus one small stretch (+1). Krashen's shorthand for the comprehensible-input sweet spot.
What it means
i+1 is shorthand. The i is your current internal grammar — the patterns and words you already have. The +1 is what's just past it: structures one step beyond what you can produce or fully analyze. Material at the i+1 level is what your brain can absorb.
Krashen never gave a precise way to measure i — and that's deliberate. The point isn't to measure. The point is the principle: don't waste time on language far above your level (you won't understand) or far below (you won't grow).
How to find your i+1
When reading, you've found it when:
- You understand most sentences without help.
- You meet a new or fuzzy word every paragraph or two — not every line.
- You can usually guess the new words from context, even if you don't always nail it.
- You feel mild effort, but the story still pulls you forward.
If you're tapping every other word, the book is too hard — drop down. If you go a whole chapter without tapping anything, it's too easy — go up.
Why it matters for reading
Most language-learning frustration comes from level mismatch. People grab a "great" book recommended in their target language, find it too hard, and conclude they're bad at the language. They aren't — they just picked something at i+5. Pick something at i+1 and the experience is completely different.