Cleft sentences

A cleft sentence splits one idea in two to stress one part: it plus be plus the stressed part plus that or who: it was Anna who called, it is the price that worries me.

A what clause stresses the end of the sentence: what I need is a holiday, what she did was call the police. The verb be agrees with what follows: what we need is time.

All works the same way for a limit: all I want is some sleep. The tense of be follows the story: it was yesterday that I saw her.