Can anyone tell the function of "that" in the following sentence?
The Mona Lisa is a beautiful painting. However, it was the mystery behind the painting that intrigued people.
In this sentence, can you identify what that is? Is that a relative pronoun or is the sentence a cleft sentence?
- It was the mystery behind the painting [that intrigued people].
As pointed out, that introduces a relative clause which is part of a construction called an It-Cleft, a transformation that makes a complex sentence out of the simple sentence
- The mystery behind the painting intrigued people.
by converting the subject into a dummy existential clause and the verb phrase into a relative clause modifying the subject. Clefts of various sorts are quite common in English; they allow speakers to indicate the relative importance of various parts of a sentence (as the speakers perceive them).
The that is here a part of the cleft construction, but it could be swapped for a wh-pronoun which. That's a part of how relative clauses work. Why that? Well, that used to mark any tensed subordinate clause. There are three kinds of subordinate clauses by function
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noun clauses like (that) he left early in I'm glad (that) he left early.
These complement clauses have an optional that at the beginning. -
adverbial clauses, which often have their own introducer, like before, as in
Before (that) we set up the tent, some of us thought we ought to eat. This that is dialectal, as shown in this answer, and represents an old construction, like the one Chaucer uses in Canterbury Tales. It's mostly not there any more in adverb clauses. -
adjective clauses, which modify nouns, like NP Complements, e.g, the rumor that the PM is a Martian, where the that works like it does in noun clauses, as well as relative clauses, where it can take the place of a wh-pronoun, even though it's officially only a complementizer (or a subordinator) and doesn't have any referent like a pronoun ought to. Over the years it's become more and more common in relatives, for one reason because it can be used even where the wh-pronoun can't, like the way (that/*how) he does it.
This is all in addition to that's role as a demonstrative pronoun (this, that, these, those). That's a busy word.
From BillJ, since it is an answer:
Cleft or not, traditional grammar classifies it as a relative pronoun, but modern grammar regards it as a subordinator.