In 1985 a linguist at the University of Southern California, Stephen Krashen, published a book that quietly reorganised his field. It had a long scholarly title and one sharp claim: people acquire a language in exactly one way, by understanding messages a little above their current level. He called it “i+1”. The i is where you are. The +1 is one notch up.
Forty years on, it's still the only piece of language-learning advice that has held up.
The trouble with +1
The trouble is that +1 turns out to be a hard quantity to deliver. Native fiction, where most learners eventually want to land, is usually more like i+50: a wall of unknown words, idiom and sentence shapes. Graded readers aim at i+0. They cover the words you already know, in stories engineered for safety. Children's books, the perennial recommendation, are often written in a deceptively complex register wrapped in friendly pictures.
The +1 is the whole trick. Get it right and you barely notice you're learning. Get it wrong and you bounce off the page.
This is why most language apps skip the reading question. Drilling vocabulary is i+0 by definition. You only ever see words you've already met, so the app gets to report mastery without exposing you to anything unknown.
What input does that vocabulary can't
Vocabulary is a list of words. A language is how those words behave in each other's company. The Spanish verb tomar means to take. It also means to drink, to have a meal, to catch a bus, to make a decision. No flashcard will teach you that. You learn it the way native speakers did, by meeting tomar in a hundred sentences and slowly noticing the shape of its world.
That kind of learning only happens through input, reading and listening, at a level your brain can just about follow. Krashen's argument was that this isn't one way to learn a language. It's the way.
What to do with that
If you've been drilling an app for months and feel stuck, the diagnosis is almost always the same. You've built a vocabulary and you haven't given it anywhere to live. The cure is reading. Stories pitched at i+1, where the words you know carry most of the weight and the few you don't are introduced gently and come back a few pages later.
That's what I'm building ReadBilingual to do. You don't need it to apply the principle, though. Find anything you can follow about 80% of without a dictionary. A comic, a children's book, a transcript. Read it twice. Then find something a hair harder.
That's it. It's unglamorous and slow, and it works.
Julia is building ReadBilingual and is, slowly, learning Chinese.