A cleft sentence is used to put emphasis on a part of the sentence by making it the subject. We use cleft sentences, especially in speaking, to connect what is already understood to what is new to the listener. In cleft sentences we usually use an IT CLAUSE and WHAT + CLAUSE. The information that follows it is what is most important for the listener.
IT + CLAUSE examples:
It is my class that has all of the challenging students in.
It was the teacher that was late, even after her specifically mentioning the importance of being on time.
We also use WHAT + CLAUSE regularly:
What made the evening so eventful was that our boss got drunk and made a fool out of himself.
Cleft sentences are case sensitive, focus on the verb to be:
What my dad needs is a long break from work. = my dad is tired now and he needs a HOLIDAY
What my dad needed was a long break from work = my dad was tired before his holiday, but now he´s OK