‘It was cats that drank up all the milk.’ is a correct sentence.
These types of sentences are called cleft sentences. Cleft sentences are used to add emphasis to what we want to say with a relative clause. If that sounds too technical, just have a look at the following examples:
- John keeps an ostrich in his garden.
- John is the person who keeps an ostrich in his garden. (emphasis on JOHN)
- An ostrich is what John keeps in his garden. (emphasis on OSTRICH)
- The garden is the place where John keeps an ostrich. (emphasis on GARDEN)
- What John keeps in his garden is an ostrich. (emphasis on WHAT)
You can also use ‘preparatory it’ to build up to what you want to say:
- It is an ostrich that John keeps in his garden.
- It is his garden where John keeps an ostrich.
- It is John that/who keeps an ostrich in his garden.
It’s also possible to emphasise a plural subject this way:
- It is ostriches that John keeps in his garden.
- It was cats that drank up all the milk.
Note: follow ‘ostriches’ and ‘cats’ with a PLURAL VERB:
- It is ostriches that love chasing people around John’s garden.
- It was cats that were lying on the sofa.
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