"Impurities"

Sorry, if it seems a stupid question, but I'd really like to learn the grammar explanation for the following, please:

"It's the impurities in our air and water that are doing this."  In this sentence why do we use It's if the word "impurity" is in the plural?

Thank you,

Saadi    

 Hi Goodman,

  Well, I understood that in fact there isn't an exact grammatic explanation why we use "It's" with plural nouns, isn't it? Is it the same as to ask for example, why the simple past and past perfect of the verb to  cut is : cut - cut. Am I wrong?

  So, when you say "It's the artificial ingredients in the foods.." We have to understand the meaning of "it's" as being in plural, in spite of writing it in the singular form, don't we?

Thanks again,

Saadi     

Anonymous

"It's the impurities in our air and water that are doing this."  In this sentence why do we use It's if the word "impurity" is in the plural?

It can't take a plural verb; in other words, you cannot say it are or it were. It is  and it was are possible. The plural impurities has nothing to do with it's but it requires the use of are later in the sentence. In sentences like the one above what comes after it is or it was is emphasized. You could also be less emphatic and say: The impurities in our air and water are doing this. More examples of similar sentences:

It was John that broke a window yesterday.
It was a window that John broke yesterday.
It was yesterday that John broke a window.

Without emphasizing anything: John broke a window yesterday.

CB 

Anonymous

"It's the impurities in our air and water that are doing this."  In this sentence why do we use It's if the word "impurity" is in the plural?

 It's a cleft sentence structure.  See Re: "It's the little things that bring me the most joy."

CJ 

Hello Cool Breeze,

Very good explanation! Thank you very much.

Best wishes,

Saadi