I used to build my own study sessions like a checklist. Twenty minutes on the past simple, then twenty on the present perfect, then twenty on phrasal verbs with “get.” Each block felt tidy. Each block felt like progress. And for years, admittedly, I recommended more or less the same approach to my students — master one thing, then move to the next.

Then I actually looked into the research on how people learn, and it turns out my tidy little blocks were quietly working against me.

Blocking vs. Interleaving

Here’s the setup. Most study plans — mine included, for a long time — follow what learning scientists call “blocking.” You pick one skill, you hammer it until it feels smooth, and only then do you move on to the next one. AAA, then BBB, then CCC.

Interleaving flips that order. Instead of finishing one topic before starting the next, you mix them together within the same session — ABC, ABC, ABC. If you’re reviewing English verb tenses, that means working through past simple, present perfect, and past continuous exercises in a shuffled, unpredictable order, rather than clearing out twenty past-simple sentences before you ever lay eyes on a present perfect one.

It sounds like a small change in order. It isn’t.

Why Mixing Things Up Actually Works

Psychologist Doug Rohrer and his colleagues at the University of South Florida have spent close to two decades testing this, mostly with math students at first. In one well-known study, students who interleaved their practice problems performed worse during the practice session itself — more hesitation, more mistakes — but roughly doubled their scores on a test given the following day, compared to students who’d blocked their practice by topic.

That gap is the whole story. Blocking gives you a comfortable, false sense of mastery, because once you know you’re only working with past-simple sentences, you stop actually having to think about which tense the sentence needs. You already know. Interleaving takes that shortcut away. Every item forces you to ask, “wait, which one is this?” before you can even attempt an answer — and that extra bit of mental friction, what researchers call “discrimination,” is what builds a memory that survives outside the practice session, out in the real world where nobody organizes their sentences by grammar point for you.

Researchers who’ve since tested this directly on language learners have found the same pattern. Studies mixing verb tenses in Romance languages found stronger long-term grammar retention from interleaved practice than from blocked drilling. A 2025 study teaching English speakers new vocabulary in German and Polish turned up the same advantage, likely tied to the natural spacing that mixing creates between repeats of the same word. The effect isn’t limited to math class. It shows up anywhere you need to tell similar things apart — which, if you’ve ever mixed up “since” and “for,” or frozen mid-sentence trying to decide between the present perfect and the simple past, is basically the entire experience of learning English grammar.

The Honest Flaw

I won’t pretend this is a free win, because it isn’t.

Interleaving feels worse while you’re doing it. You’ll make more mistakes in the moment. Your sentences will come out slower and clumsier than they would if you were drilling one structure in isolation, and that can be discouraging if you’re not expecting it going in. One study on learners practicing French grammar found that the interleaved group ended up more accurate than the blocked group — but less fluent, at least in the short term. The blocked group sounded smoother. They were also more likely to be confidently wrong in ways that stuck around.

There’s a second catch, and it matters more for beginners. Interleaving works best as a review strategy, not a first-contact one. If you’re meeting a grammar structure for the very first time, research on lower-level learners suggests you need a bit of blocked repetition first, just to get your footing, before mixing it in with everything else starts to pay off. Throwing five unfamiliar structures at yourself in random order on day one isn’t interleaving. It’s just chaos with extra steps.

So get comfortable with something new first. Then start mixing it into your review.

How to Actually Use This

None of this requires an app, a subscription, or a new notebook system. It requires you to stop organizing your study time so neatly.

• Shuffle your grammar review. Instead of a worksheet of twenty present-perfect sentences, pull together a mixed set — present perfect, past simple, and past continuous, thrown in together — so you have to identify the tense before you can use it correctly.
• Rotate the pairs that confuse you most. “Since” vs. “for.” Comparatives vs. superlatives. The first and second conditional. Study these side by side rather than mastering one before the next begins — the confusion between them is exactly what you’re training your brain to resolve.
• Mix your vocabulary themes. Don’t memorize twenty travel words in one sitting and twenty food words in the next. Shuffle them together. Your brain forms stronger, more flexible connections when it has to sort unrelated items rather than glide through a predictable list.
• Alternate skills, not just topics. A session that moves between reading, a short listening clip, and a few minutes of speaking practice will serve you better than an hour of pure reading, even if the reading-only hour feels more focused while you’re in it.
• Work in short, shuffled blocks. Fifteen to twenty minutes per topic, reordered across sessions, spread out over several days rather than crammed into one long sitting. You’re not trying to finish a topic. You’re trying to keep bumping into it from a different angle each time.

Interleaving vs. Spaced Repetition — Not the Same Thing

These two get mixed up constantly, so it’s worth pulling them apart. Spaced repetition is about when you review something — a little today, a little more in three days, a little more in a week, stretching the gaps as the memory sticks. Interleaving is about what order you review things in during a given session — mixing topics instead of blocking them. They’re not competing strategies. A well-built study routine uses both: spaced-out review sessions, and within each one, a shuffled mix of topics instead of a single block.

The Takeaway

Comfortable practice and effective practice are not the same thing, and English learners chase the first one far more often than they should. If a study session feels smooth and easy from start to finish, you’re probably not building anything that will survive contact with a real conversation.

Go mix something up today. Pull three grammar points you already sort of know, throw them into the same practice set, and force yourself to tell them apart. It’ll feel worse before it feels better. That’s the whole point.