Halliday & Matthiessen (2014: 95-6):
A thematic equative (which is usually called a ‘pseudo-cleft sentence’ in formal grammar) is an identifying clause which has a thematic nominalisation in it. Its function is to express the Theme-Rheme structure in such a way as to allow for the Theme to consist of any subset of the elements of the clause. This is the explanation for the evolution of clauses of this type: they have evolved, in English, as a thematic resource, enabling the message to be structured in whatever way the speaker or writer wants.Let us say more explicitly what this structure means. The thematic equative actually realises two distinct semantic features, which happen to correspond to the two senses of the word identify. On the one hand, it identifies (specifies) what the Theme is; on the other hand, it identifies it (equates it) with the Rheme.The second of these features adds a semantic component of exclusiveness: the meaning is ‘this and this alone’. So, the meaning of what the duke gave my aunt was that teapot is something like ‘I am going to tell you about the duke’s gift to my aunt: it was that teapot — and nothing else’. Contrast this with the duke gave my aunt that teapot, where the meaning is ‘I am going to tell you something about the duke: he gave my aunt that teapot’ (with no implication that he did not give – or do – other things as well).Hence even when the Theme is not being extended beyond one element, this identifying structure still contributes something to the meaning of the message: it serves to express this feature of exclusiveness. If I say what the duke did was give my aunt that teapot, the nominalisation what the duke did carries the meaning ‘and that’s all he did, in the context of what we are talking about’.This is also the explanation of the marked form, which has the nominalisation in the Rheme, as in that’s the one I like. Here the Theme is simply that, exactly the same as in the non-nominalised equivalent that I like; but the thematic equative still adds something to the meaning, by signalling that the relationship is an exclusive one – I don’t like any of the others. Compare a loaf of bread we need and a loaf of bread is what we need. Both of these have a loaf of bread as Theme; but whereas the former implies ‘among other things’, the latter implies ‘and nothing else’.Note that some very common expressions have this marked thematic equative structure, including all those beginning that’s what, that’s why, etc.; e.g. that’s what I meant, that’s why it’s not allowed.
