Can I use "were" in this sentence?

Question

My sentence is,

"I thought it were your friends (that) you went out with."

I used the be-verb were in this sentence because it refers to your friends.

Am I right?

Answer 1

No, you need to use was there because the subject of the sentence, it, is grammatically a singular entity. Linguists call this type of it an impersonal subject. It semantically has no meaning. It's only there for the sake of grammatical completeness.

Examples:

I thought it was you who called me last night.
I thought it was your friends that you went out with.

Answer 2

This kind of sentence is known as an 'it-cleft'. The it-cleft emphasizes the element that "it" is equated with:

I thought it was your friends you went out with (not your mother).

I thought it was the red one you wanted (not the blue one).

But the verb will always be singular, "was". "it was", "it is".