“Cleft” means divided. Cleft sentences can be used to emphasize a particular part of a sentence. In a cleft sentence, one element of the sentence is shifted from its normal place in the sentence into a separate clause. The emphasis in the sentence changes in a cleft sentence, depending on what is shifted.
Cleft sentences are particularly useful in writing where we cannot use intonation for purposes of focus or emphasis, but they are also frequently used in speech.
What
We frequently emphasise an action or activity with What + happen + (that) + clause.
What happened was that we had to throw all the food away.
What happened was that their car broke down on the motorway so they didn't get to Mark's wedding on time.
We can emphasise actions with What + noun phrase + do / does /did + be + (to) infinitive.
What the police did first was (to) interview all the witnesses to the accident.
What she does is (to) write all her novels on a type writer.
What Mary had done was (to) get married.
All
We can begin cleft sentences with All.
All I did was (to) touch the bedside light and it broke.
All she wants is a new phone.
The thing / One thing ..
We can begin cleft sentences with The first thing, the main thing ...
The main thing is that you two stop quarrelling.
One thing you can do is to promise it won't happen again.
The place where / time when / reason why
We use place, where, tine when and reason why to emphasise a place, time or reason. We can use that instead of where, when and why.
The place (where) I was born is Madrid
The day (when) the Second World War ended in Europe was 7 May 1945
The reason why I've come is to discuss my future with you.
It cleft sentences
We can also use preparatory it in cleft sentences and join the words that we want to focus on to the relative clause with that, who, which or when.
Imagine that my brother bought his new car from our next-door neighbour last Saturday. Depending on what we want to focus on we can say :
It was my brother who bought his new car from our neighbour last Saturday.
It was last Saturday when my brother bought his new car from our neighbour.
It was a new car that my brother bought from our neighbour last Saturday.
It was our next-door neighbour that my brother bought his new car from last Saturday.
Another example, imagine Max asked the teacher a silly question after school.
You can emphasize the subject: It was Max who asked a silly question about math after school.
You can emphasize the object: It was a silly question that Max asked about math after school.
You can emphasize the adverbial: It was about math that Max asked a silly question after school.
You can emphasize the prepositional phrase: It was after school that Max asked a silly question about math.
We can also emphasise time with It is/ was not until...that and It is/was only when ... that
It was not until Sarah went away that I realised how much I missed her.
It's only when I'm alone thatI feel insecure.