The pseudo-cleft sentence is a device whereby, like the cleft sentence proper, the construction can make explicit the division between given and new parts of the communication. It is essentially an SVC sentence with a nominal relative clause as subject or complement. . . .
“ subject+linking verb+complement
i.e. linking verb=connects the s+complement=describes the subject
e.g I am sexy. The bread smells good.”
The pseudo-cleft sentence occurs more typically with the wh-clause as subject, since it can thus present a climax in the complement:
Wh-clauses are what, when, where, who, which, why and how.
“What you need most is a good rest.”
It is less restricted than the cleft sentence . . . in one respect, since, through use of the substitute verb ‘do’ (in the following examples), it more freely permits marked focus to fall on the predication:
What he’s done is (to) spoil the whole thing. What John did to his suit was (to) ruin it. What I’m going to do to him is (to) teach him a lesson. In each of these, we would have an anticipatory focus on the do item, the main focus coming at normal end-focus position.” (Randolph Quirk, Sidney Greenbaum, Geoffrey Leech, and Jan Svartvik, A Grammar of Contemporary English. Longman, 1985)
“What is striking is that the wh-clause of the pseudocleft anticipates (or ‘projects’) up-coming talk by the same speaker, and . . . FRAMES that talk in terms of such categories as event, action and paraphrase.”