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I don't understand why the author used "It is" rather than They are in the following sentence.

It is threads, not processes, that are the units scheduled by the system for execution on the processor.

Why don't we say the following?

They are threads, not processes, that are the units scheduled by the system for execution on the processor.

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This is a cleft sentence: "It is noun phrase that clause." In this construct, the pronoun is always "it"; it doesn't agree with the number (or gender) of anything. So it doesn't matter that threads or are are plural.

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    I agree with this answer, and a cleft sentence is one type of use of dummy pronouns. I think the only dummy pronouns in English are "it", "there", and the "wh-" relative pronouns, but never "they". – aschepler Apr 29 '19 at 12:20
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I would say that "It is" is definitely correct because "they are" totally changes the meaning. But I am struggling to say why.

The problem really is the use of the plural "threads". So a better version might be:

It is the thread, not the process, that is the unit scheduled ...

But that loses the fact that there are multiple threads that are scheduled.

Better:

It is the set of threads, not processes, that provide the units scheduled ...

So maybe we should consider that "threads" is being used as an uncountable noun (for the concept of threads) that just happens to look like a plural.

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    It is oranges I like, not apples. That is clearly plural, and yet the singular verb makes sense. (The subject is it, not the plural things.) In this case, I think that the singular can apply, regardless of the plurality of the object. I believe it's a dummy pronoun, as in it is raining. We could also say it is many small things that can drive somebody to distraction. – Jason Bassford Apr 29 '19 at 7:55

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