Fui or iba? Every learner of Spanish knows this moment: two past tenses, and a sentence that needs exactly one of them. In 2019, the psychologist Steven Pan and four colleagues published four experiments built around this choice. They taught adults to conjugate Spanish verbs in the preterite and the imperfect. One group practised one tense at a time. The other group practised both tenses mixed together in random order.
That second approach is called interleaving: mixing different types of problems in one practice session instead of finishing one type before starting the next. For grammar choices like these two tenses, it worked, but only when practice was spread over more than one session. For learning plain vocabulary, the evidence points the other way. Here is what the studies found, and how to use the idea without wasting time.
The Spanish verb experiments
Pan's team published the work in the Journal of Educational Psychology. In the first two experiments, all training happened in a single session. Mixing the tenses did not help. When mixing started right from the first explanation of the two tenses, the blocked group's scores two days later were, if anything, slightly higher. When mixing came only in the practice at the end, the two groups did equally well.
In the third and fourth experiments, training was spread over two sessions a week apart. The blocked group practised one tense in each session. The interleaved group practised both tenses in both sessions, mixed at random. On a test one week later, the interleaved group did substantially better. The authors called it the first demonstration of an interleaving effect for foreign language learning.
A second study, with English grammar
That same year, Tatsuya Nakata and Yuichi Suzuki tested the idea with 115 Japanese learners of English, who practised five English grammar structures (The Modern Language Journal). Some practised one structure at a time, some mixed them all, and some started with blocks and then mixed.
During training, the mixed group made the most mistakes. One week later, it beat the blocked group. There was one more detail: learners who knew less at the start gained more from mixing than those who already knew more.
Why mixing helps with grammar
When you practise one form at a time, you know which rule to use before you even read the question. The exercise only asks you to apply it. Real language never gives you that hint. In a conversation, nobody tells you that the next sentence needs the imperfect. Mixed practice forces you to decide which form fits, and that decision is the actual skill.
A large meta-analysis by Brunmair and Richter, which combined 59 studies, supports this picture. Interleaving helped most when the categories were similar to each other and therefore easy to confuse. Two Spanish past tenses, or der, die and das, fit that description well.
Why it feels worse
Mixed practice is slower and produces more errors, so it feels less effective while you do it. A classic 2008 study by Kornell and Bjork showed how strong this feeling is. People learned to recognise painters' styles, either one artist at a time or with artists mixed. Mixing worked better. Yet participants rated the one-at-a-time method as more effective, even after their own test results had shown the opposite.
So if mixed practice feels clumsy, that is not a sign it is failing. Judge it by how you do next week, not by how smooth today's session felt.
Where interleaving doesn't help
The same meta-analysis found a moderate effect overall, strongest for paintings and other visual material and smaller for maths problems. For studies that used words as the learning material, the result went the other way: blocking did better than interleaving. The authors advise caution with interleaving for words and for expository texts, the kind that explain facts.
The honest summary for language learners: mixing is a tool for practising choices between forms you have already met. It is not a trick for memorising new vocabulary faster.
What this means for you
- Learn each new form on its own first. In Pan's study, mixing from the first explanation did not help. Understand the preterite, understand the imperfect, then mix.
- Then mix the forms that compete. Spanish preterite and imperfect, Italian passato prossimo and imperfetto, Polish perfective and imperfective verbs, German articles and cases.
- Spread the mixed practice over several days. The benefit appeared across sessions a week apart, not within one sitting.
- Expect more mistakes during practice. Mistakes are part of having to choose. Check your progress a week later.
- Leave new word lists alone. For memorising new vocabulary, regular spaced review does the work. Our guide to spaced repetition explains how to time it.
Qlango's review schedule gives you the "more than one session" part. A finished lesson comes back after one, two, four and eight days and then once more, so each grammar point returns in several sessions spread over weeks. When several lessons from different themes are due on the same day, repeating them one after another mixes them in a single sitting. And because you build sentences and write answers yourself, you have to choose the form, not just recognise it.
Fui or iba? You can't answer that question by drilling one tense for an hour. You answer it by meeting both, mixed, on more than one day, until choosing becomes the easy part. Our comparison of vocabulary learning methods covers the tools for the words themselves. Our plan for learning a language on your own shows where mixed grammar practice fits in a week.
Frequently asked questions
What is interleaving?
Interleaving means mixing different types of problems or topics within one practice session, instead of practising one type until it is finished (called blocking). In language learning, an example is practising two past tenses in random order rather than one after the other.
Is interleaved or blocked practice better?
For learning to tell similar things apart, interleaved practice is usually better on later tests. A meta-analysis of 59 studies found a moderate overall advantage. In language studies, interleaving helped with Spanish past tenses and English grammar structures on tests a week later. In the Spanish study, though, it did not help when all practice happened in one session.
Does interleaving work for vocabulary?
The evidence says no, or not yet. In the meta-analysis by Brunmair and Richter, studies that used words found an advantage for blocking instead. Use interleaving for grammar choices and spaced review for new words.
Why does interleaving feel harder?
Because you have to decide which rule or form applies before you can use it, which is slower and leads to more errors. Studies show people tend to rate blocked practice as more effective even when their own test results show the opposite.
