An it-cleft sentence takes a single idea and splits it into two clauses to put a spotlight on one piece of information. The basic frame is:

It + be + focused element + that/who/which clause

Take an ordinary sentence: Maria fixed the printer on Monday. You can cleft it several ways depending on what you want to emphasize:

• It was Maria who fixed the printer on Monday. (focus on the person)
• It was the printer that Maria fixed on Monday. (focus on the object)
• It was on Monday that Maria fixed the printer. (focus on the time)

Notice that the rest of the sentence — everything that isn't spotlighted — goes into the clause after that/who/which. The structure itself isn't hard, but two areas cause real trouble at B1–B2 level: subject–verb agreement and clefting adverbials.

Agreement: which verb agrees with what?

The verb be after it agrees with the tense and context, not with the focused element. It is always singular, so you write it was, it is, never it were (unless you're in a subjunctive context, which is rare here). That part is straightforward.

The trickier agreement happens inside the relative clause. Look at these two sentences:

✅ It is the students who are responsible for the project. ❌ It is the students who is responsible for the project.

The verb in the relative clause (are) must agree with the focused noun (the students), not with it. Many learners automatically write is because their eye is still on It is… at the start. Train yourself to look at the noun right before who/that — that noun controls the verb that follows.

One more pair to compare:

✅ It was I who was late. (formal; some speakers accept who were late by analogy with I were in subjunctive, but in standard clefts, was is normal) ❌ It was me who were late.

In everyday English you'll hear It was me who was late, with the object pronoun. Exams and formal writing prefer I, but either way the relative-clause verb stays was because the reference is to a single person.

Clefting adverbials

You can focus not just on nouns but on adverbial phrases — of time, place, reason, or manner. When you do this, the connector is almost always that, not when, where, or why:

✅ It was in 1969 that humans first landed on the Moon. ❌ It was in 1969 when humans first landed on the Moon.

The when version feels logical ("1969 is a time, so use when"), and that's exactly why so many learners choose it. But in a standard it-cleft the relative word is that — or you can drop it entirely: It was in 1969 humans first landed on the Moon. The same principle applies to place and reason:

✅ It was at the café that we met, not at the library. ❌ It was at the café where we met.

The mistake to watch for

The single most common error is mixing up two jobs at once: using when/where inside an it-cleft and breaking agreement. A sentence like It is on weekends when the shops is crowded has both problems. Fix it in two steps: switch when → that, then check agreement (the shops → are):

✅ It is on weekends that the shops are crowded.

A quick self-check

Before you move on to the quiz, try clefting this sentence three different ways — focusing on the person, the object, and the time:

Tom sent the email yesterday.

If you got It was Tom who sent the email yesterday, It was the email that Tom sent yesterday, and It was yesterday that Tom sent the email, you're ready.