Cleft means divided in two. A cleft sentence, then, is a sentence in which information which could be given in one clause is divided into two parts. This allows the speaker to give added emphasis to certain bits of information. Cleft sentences typically begin with “it” or “it was.”

So the sentence “John took Mary to the concert last week,” could be made into a cleft sentence in a few different ways and each would emphasize different pieces of information.

It was John who took Mary to the concert last week.

It was Mary that John took to the concert last week.

It was to the concert that John took Mary last week.

It was last week that John took Mary to the concert.