 A cleft sentence is a complex sentence where special emphasis is put on any element of clause structure, except the verb, by means of a specific main clause construction. Syntactically, clefts are introduced by a main clause as an intensive construction, with an impersonal subject it, an inflected intensive verb, plus a subject complement, post-modified by a subsequent subordinate clause introduced by that, or an alternative WH element. Let's illustrate this using the simple sentence, yesterday John painted the kitchen dark green, where yesterday is an adverbial, John the subject, the kitchen the object, and dark green an object complement. Let us now successively put all elements of clause structure, except the verb, into focus. If the adverbial yesterday is put into focus and becomes the subject complement of it, we get, it was yesterday, that or when John painted the kitchen dark green. In, it was John who painted the kitchen dark green, the subject is in the focus and becomes the complement of it. If the focus is on the object we get, it was the kitchen that John painted dark green and the kitchen is our subject complement. And even the object complement can stand in the focus as in, it was dark green that John painted the kitchen. Subject complements as focus of clefts are more restricted, especially with a form of B at the end of the subordinate clause. It is a doctor that he is, is ungrammatical. But without these restrictions they work. It was a doctor that he became, well that works. So cleft sentences are flexible devices to highlight different parts of sentences in present day English.