There are basically two types of cleft sentences – the it-type and the wh-type (usually introduced by "what"):What we need is more time.
It-clefts consist of:
• the pronoun it
• a form of the verb be, optionally accompanied by the negation not or an adverb such as only
• the specially focused element (a noun phrase, a prepositional phrase, an adverb phrase, or an adverbial clause)
• a relative-like dependent clause introduced by that, who/which, or zero, whose last element receives normal end-focus.
The cleft sentence with "it" reverses the usual order of "given / old information – new information" by presenting the new – and important – information first.
Wh-clefts consist of:
· a clause introduced by a wh-word, usually what, with its own point of focus, typically at its end
· a form of the verb be
· the specially focused element (a noun phrase or an infinitive clause)
It-clefts are relatively common in all registers but most frequent in academic prose, while wh-clefts are most frequent in conversation.