What is comprehensible input (i+1)? How do I pick anime and books at the right difficulty?

Two terms come up all the time in immersion learning: "comprehensible input" and "i+1". Both answer the same question: what should I watch or read to actually learn something? Content that's too easy teaches you nothing new because you already understand all of it; content that's too hard teaches you nothing either, because you can't follow a single line. This page explains the idea itself and then makes it practical — how to choose a show or book at the right difficulty.

What does comprehensible input mean? ​

The term comes from linguist Stephen Krashen's "input hypothesis". The core idea is simple:

"We acquire language in only one way: by understanding messages."

In other words, a language isn't "learned" by memorising rules — it's "acquired" in the process of understanding messages. As a child you never memorised your native language's grammar tables, yet you speak it more naturally than any grammar book; what got you there was a massive amount of input you mostly understood.

The key word is "comprehensible". A foreign-language film you don't understand at all teaches you very little even after a hundred viewings: you can't tell what's being said, so your brain has nothing to connect. But when you understand most of a sentence and only one or two bits are new, context, picture and tone help you guess those bits — and that guess is where acquisition happens.

What is i+1? ​

"i" is your current level, "+1" is a little above it. i+1 input is input you can mostly understand at your current level, with a small amount of new material in it.

To be clear: this is a way of thinking about choosing material, not a formula you can measure. Nobody can calculate what your "i" is right now, and no show is exactly "+1". In practice, treat it as a direction: pick content you can keep watching, but don't fully understand.

For mining there's an especially handy version of i+1: the sentence with exactly one unknown word. You can already read everything in it except that word, and the context explains the word for you — the best possible card material. Why cards should carry the original sentence is covered in What is sentence mining?

How do I tell if a show or book is the right difficulty? ​

There's no precise test, but there are useful signals:

Probably too hard:

• Without helper subtitles, you have no idea what happened in an episode;
• every line needs several lookups, and you still don't get it afterwards;
• after ten minutes you feel tired and want to watch something else;
• you notice you're "studying" rather than "watching".

Probably about right:

• You follow the general plot, and some of the lines you hear or read make sense;
• there are plenty of unknown words, but you can keep watching without looking them up, and lookups make things clear;
• when an episode ends, you want the next one.

Probably too easy:

• You understand nearly every line and rarely meet a word worth looking up;
• it's still great input (for listening and speed), but you can start moving to harder content.

The most important factor isn't actually difficulty — it's interest. A slightly-too-hard show you really want to watch often works better than a perfectly pitched one you don't care about, because you'll keep watching. As the immersion guide says: choosing content you like matters more than anything. You can lower difficulty with helpers; you can't manufacture interest.

How does content roughly rank from easy to hard? ​

These are general tendencies, not rules — works within one genre vary a lot:

• Rewatching something you've seen in your own language: you know the plot, so it's naturally a notch easier — an excellent choice when starting out.
• Slice-of-life, school and everyday shows: the dialogue is mostly everyday conversation with common vocabulary and situations.
• Anime for young children: slower and simpler, though not necessarily to your taste.
• Battle, fantasy, sci-fi, mystery and historical works: lots of specialised vocabulary (techniques, world-building, jargon, archaic language), usually harder.
• For reading: manga gets help from the pictures and is a good first step; light novels are generally easier than literary fiction; voiced visual novels train listening and reading together.

You can also adjust the difficulty of the same work by how you watch it: Japanese subtitles are easier than none, and a blurred native-language secondary subtitle catches you when you're stuck. Stage-by-stage advice on subtitles is in Japanese or native-language subtitles?

How do I use comprehensible input with Fushi? ​

What Fushi does is pull content that's a bit too hard for you back into the comprehensible range:

• Tap to look up: words in subtitles, novel text and manga (via OCR) can be tapped for their definitions, turning "that one word I didn't get" into one you do. If you're looking things up constantly, every line, that's a sign the content is on the hard side.
• Use frequency to decide what's worth keeping: with frequency dictionaries installed (the recommended pack in onboarding includes Japanese ones), the lookup popup shows each word's frequency. Common words are worth a card; very rare ones can wait — they'll turn up again in a better place. More on reading frequencies in What is word frequency?
• Mine only i+1 sentences: on a sentence that's "just this one word short", tap the plus to make a card with the sentence, the scene and the original audio; skip sentences you don't understand at all.
• Reorder new cards by frequency: "Reorder new cards by frequency" in Settings → Card creation puts common words first (AnkiConnect only), so your "i" grows first across the most frequently used words.
• Tune difficulty with a secondary subtitle: put your native language in the secondary subtitle and blur it, glancing only when completely stuck — see How do I use a secondary subtitle as a bridge?
• Audiobook plus text: a book you struggle to read becomes easier when you listen to its audiobook while following the text, compared with reading or listening alone — see Audiobooks.

Common misconceptions ​

• "I'll wait until my vocabulary is big enough for real content": however much you prepare, you won't understand everything the first time you meet real content. Comprehensible input doesn't require full understanding — mostly is enough. You can start once you're partway through a basic deck.
• "Harder means faster progress": input too hard to understand gives the slowest progress of all, and is the most likely to make you quit.
• "I have to understand every word": you don't. For many words a rough guess from context is enough, and they get clearer the next time. Tolerating ambiguity is one of the skills immersion trains most.
• "Comprehensible input means only listening, no studying": lots of input is the main track, but active tools — lookups, basic vocabulary, mining cards — make more content comprehensible sooner. The two work together.

For the full starting route, see the immersion guide and Can you learn Japanese from anime?