A couple of years ago I posted a lesson on cleft sentences where I said:
The structure is:
It is / was + emphasised noun phrase + who / that / when + rest of the sentence
Notice that the use of pronouns is the same as in defining relative clauses:
- who or that for people
- that (NOT which) for things and after prepositional phrases
- As in defining relative clauses, who and that can be left out when they refer to the object or the object of a preposition.
I'll quickly gloss over the fact that I compared using that and not which to the use of pronouns in defining relative clauses (where, of course, we can use which, despite the naysayers).
The problem was that a commenter, a certain elhamcz, had noticed that while I had ruled out which for things, one of the resources I had linked to included it in their list of allowable pronouns, and not surprisingly elhamcz was rather confused. Furthermore elhamcz wanted to know what other relative pronouns or wh-words, for example whom and where, could be used in it-clefts, and whether there were any sources that could provide an answer to this problem.
Now I am neither a linguist nor a grammar expert, and had based my lesson on EFL grammars that I use regularly, but I wondered if perhaps I was being too categorical in dismissing which for things, and should I have included when? Anyway, I decided to have a root around.