Isn't it, or are they?
 

Excuse the silly question, but I can't give another answer other than 'because it just is'. I was asked to about the following: "Isn't it aeroplanes that are mainly responsible for the pollution in the atmosphere?".

I guess the main reason for the question is, why is "it" used for earoplanes (plural), instead of 'are'? My reply was that it is a question and not a statement.

Can anyone help with this, in more technical term. Or, is it really as simple as 'because it is?'

Re: Isn't it, or are they?
 

I wish I could help, but exactly the same thing is bothering me too...

Re: Isn't it, or are they?
 

It is odd, but your explanation wasn't quite right. It's nothing to do with questions and statements.

This is what is known as a cleft-sentence, using anticipatory-it. Cleft sentences are used to give emphasis - compare :

I want to see the other film
It's the other film which I want to see

The second (the cleft sentence) highlights the topic by making it the complement of it+be with a relative clause following. Be can be in any form (eg It was the other film which I wanted to see) and affirmative, negative or interrogative (It wasn't Peter that I was talking about: Is it this book which you want to read? Wasn't it the other film which you wanted to see?)

So that's what's going on - so far so good. The apparently odd thing, as you point out, is that it doesn't change for the plural :

Aeroplanes are mainly responsible ... etc still ends up as :
It's aeroplanes which are etc

Debate rages as to why, and some of the linguistic explanations are extremely complex. The simplest (and to my mind probably correct) explanation is simply that It+be doesn't in fact refer to the topic of the sentence (here aeroplanes) but instead is introductory to the whole idea. Compare sentences like It is easy to see why this area is so confusing - where you again have this kind of introductory it+be.

Certainly not a silly question though.