Spaced repetition (SRS) for language learning

A spaced repetition system (SRS) is flashcard software that decides when you see each card again. Cards you know well drift weeks and then months into the future. Cards you keep forgetting come back tomorrow. The result is that most of your review time goes to the items that need it.

For languages with thousands of characters to hold on to, this is close to indispensable. It is also the easiest study tool to misuse, mostly by adding too much and by treating the review queue as the language itself.

How spaced repetition works

Two findings from memory research do the work.

The first is the forgetting curve. Hermann Ebbinghaus showed in 1885 that newly learned material fades quickly at first and more slowly later. The second is the spacing effect: the same number of reviews produces more durable memory when they are spread out than when they are crammed together. Recalling something just as it is starting to fade strengthens it more than recalling it when it is still fresh.

An SRS turns this into a schedule. After each successful review, the gap before the next one grows. After a failure, it shrinks. In the 1970s Sebastian Leitner did this with a box of paper cards divided into compartments. Piotr Woźniak's SuperMemo put it into software in the 1980s, and his SM-2 algorithm became the basis for most apps that followed.

One more ingredient matters: the card makes you retrieve the answer before you see it. Re-reading a word list feels productive and does little. Being asked and having to answer is what makes the memory stick.

Is Anki good for learning languages

Anki is free on desktop and Android, open source, and endlessly configurable, which is both its strength and the reason people spend a weekend on fonts. For language learners it is the default choice when you want to control what is on your cards, especially for sentence mining.

It is not the only option:

• Fixed-curriculum apps such as WaniKani for kanji run their own SRS on a set syllabus. You give up control and gain a path someone else has planned.
• Dictionary apps with flashcards, Pleco for Chinese being the best known, let you send a looked-up word straight to a review queue.
• Reading apps with built-in review. Du Chinese lets you save words while you read graded stories and reviews them with its own spaced repetition flashcards. Mandarin Mosaic builds the whole course on it: sentences made of words you know plus one new word, scheduled for review.
• Paper cards in a Leitner box still work. They cost almost nothing and cannot send notifications.

The tool matters less than two habits: reviewing daily, and being strict about what goes in.

SM-2 and FSRS: what changed

For most of its history Anki scheduled cards with a modified version of SM-2. Since version 23.10 it also offers FSRS, the Free Spaced Repetition Scheduler, developed by Jarrett Ye. The difference is worth understanding because it changes which settings you should touch.

	SM-2 (Anki's older scheduler)	FSRS
What it tracks per card	An interval and one "ease" number	Three values: difficulty, stability and retrievability
How the next interval is set	Previous interval multiplied by the ease	The point at which predicted recall falls to your target
Adapts to you	Only through your button presses on each card	Parameters are fitted to your whole review history
Main control	Several multipliers and step settings	One number: desired retention
After a long break	Less accurate with overdue cards	Schedules delayed reviews much better, according to Anki's documentation

In FSRS terms, retrievability is the chance you can recall a card right now, stability is how many days it takes for that chance to fall from 100% to 90%, and difficulty describes how hard it is to make the card more stable. You choose a desired retention, and the scheduler shows each card when its predicted recall has dropped to that level.

The practical upshot, as stated in Anki's own FAQ, is that FSRS needs fewer reviews than the older algorithm for the same retention. The FSRS project's simulations put the saving at roughly 20 to 30%. Treat that figure as the developers' estimate, though the direction is not in dispute.

SM-2 has a known weak spot that FSRS removes: every "Again" lowers a card's ease, and nothing raises it back unless you press "Easy". Hard cards can end up reviewed far more often than necessary. Anki users call this ease hell, which is dramatic but fair.

Sane Anki settings

These follow the Anki manual's deck options page and are deliberately conservative.

• Turn on FSRS in the deck options if it is not already on. It applies to the whole profile.
• Desired retention: leave it at 90%. The manual warns that workload rises quickly above 90% and can become overwhelming above 97%. Lower it to 85% if reviews are crowding out everything else.
• Learning steps: short and few. With FSRS, keep all steps under one day, for example a single 10-minute step. Multi-day learning steps fight the scheduler.
• Optimise the parameters about once a month, once you have at least a few hundred reviews. Before that the defaults are fine.
• Maximum reviews per day: set it high. A low cap does not reduce what is due. It only hides it. Control workload with new cards instead.
• Use "Again" when you forgot. Pressing "Hard" on a card you failed tells FSRS you remembered it, and intervals grow far too long.
• Leave "reschedule cards on change" off when you first switch from SM-2, or a large pile of cards may come due at once.

How many new flashcards to add per day

Anki's default is 20 new cards a day, and the manual's own rule of thumb is that this leads to roughly 200 reviews a day once the deck matures. In other words, expect daily reviews of about ten times your new-card number.

Work backwards from the time you have:

Daily SRS time you can sustain	New cards per day (rough guide)
10 minutes	5
20 minutes	10
40 minutes	15 to 20

These are Kungfucius's estimates for simple word or character cards. Sentence cards take longer per review, so halve the numbers. And "sustain" means on a bad day in February, not on the first enthusiastic Sunday.

New cards are a loan against future time. Twenty a day feels free for a week, and the repayments start in week three.

Everything not mentioned here can stay on default. The urge to tune further usually means it is time to close the settings and do the reviews.

What to put on cards

• One thing per card. A card that asks for meaning, reading and three example uses will be failed for a different reason each time.
• Things you have already met. Cards made from your own reading and listening stick far better than a downloaded list of strangers. Premade decks are reasonable for the very first words and characters, when you have no input to draw from yet.
• Sound. For Mandarin tones or Japanese pitch, a card without audio trains half the word. Learn the pronunciation before or with the card, never afterwards.
• Characters with a purpose. For Chinese and Japanese, an SRS is a practical way to build a base of the most common characters before reading is comfortable. Graded readers become approachable somewhere in the first several hundred.

Common mistakes with spaced repetition

Making SRS the main activity. Flashcards keep words from leaking out. They do not teach you to follow a conversation. As a rule of thumb, keep reviews to a fifth of your study time or less and spend the rest on input and speaking.

Adding faster than you can review. The backlog arrives with a delay, which is why it surprises people.

Skipping days. Reviews pile up, and the pile is discouraging. If you do miss a week, set new cards to zero and work the backlog down over several days. FSRS copes with the overdue cards sensibly.

Keeping bad cards out of duty. A card you fail for the eighth time is badly made or not yet useful. Rewrite it, suspend it or delete it.

Studying two languages in one deck. Mixed decks invite the confusions described under language interference. Give each language its own deck and, ideally, its own time of day.

What to do this week

1. Install Anki or pick one of the alternatives above, and commit to one tool for a month.
2. Apply the settings in this guide, and set new cards to 10 a day or fewer.
3. Add only words or sentences from what you read or heard this week.
4. Review every day, before anything else. Ten minutes is enough to start.
5. After two weeks, look at your daily review count and adjust new cards up or down by a few. Then leave the settings alone and go and read something.