Spaced Repetition for Language Learning

Spaced repetition is the closest thing we have to a cheat code for vocabulary. Here's how it actually works, why most apps get it wrong, and how to use it without burning out.

What spaced repetition actually is

In 1885, Hermann Ebbinghaus tested how fast he forgot nonsense syllables. The curve he drew — a steep drop in the first hour, a gentler slide after — is still the foundation of every modern study app. The insight that followed, confirmed by a century of research, is simple: if you review a fact right before you're about to forget it, the next interval until you forget it gets longer. Review it again at that edge, and the interval stretches again.

That's spaced repetition. Each card has a memory strength, each correct review extends the interval, each mistake resets it. Do it right and you can hold thousands of words in long-term memory for a few minutes of daily work.

SM-2, the algorithm under the hood

CodexTutor uses the SM-2 algorithm — the same family that powers Anki, SuperMemo, and most serious SRS tools. Every card tracks an ease factor and an interval. When you rate a review (again, hard, good, easy), the algorithm adjusts both. Struggle with a card and it comes back in a day. Nail it three times in a row and you won't see it for a month.

The math isn't magic — what makes it work is discipline on the input side. Feed the deck words you actually need and it sings. Feed it random vocabulary lists from a textbook and you'll memorize a lot of words you'll never use.

Why flashcard-only apps plateau

Anki can hold 10,000 cards. Most users quit at 500. The reason is almost never laziness — it's that grinding isolated words is demoralizing. You're memorizing a list, not reading a language. Worse, words memorized in isolation tend to stay isolated: you recognize fleuve on a flashcard but freeze when you hit it in a sentence.

This is where most language learners stall. They know 2,000 words in Anki and still can't read a page of a novel.

Pairing SRS with real reading

The fix is to flip the order. Instead of drilling words first and hoping to meet them later, meet them in a book first, then review them.

CodexTutor does this automatically. When you read a chapter in Study Mode, every word you encounter is added to your "Next Up" queue. SRS review pulls from three tiers:

Review — cards that are due today based on SM-2
Next Up — words you've actually encountered in your reading, weighted by frequency
Looking Forward — high-frequency words in texts you haven't started yet

Every review is a word you've already seen in context, attached to a real sentence by a real author. That's the difference between "I think this means…" and "I know this word — it's the one Perrault used for the wolf."

Productive mode: the final layer

Recognizing a word is easier than producing one. After you've successfully recalled a word for seven days straight in the receptive direction (target → English), CodexTutor unlocks it in the productive direction (English → target). This is where passive knowledge becomes active output.

A sustainable daily practice