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I'm confused what verb would concord in the following couple of sentences:

The number of students and the number of teachers are/is decreasing day by day in the school.

Either a number of teachers or a number of students is/are participating in the college's festival.

According to my understanding the number of is always followed by a singular verb even two the number of are joined with and. Also, a number is always followed by a plural verb. So I think the former will take is while the latter concord with are. But I'm not sure.

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The first one is very awkward. If this is a real, specific example from something you're writing, I'd suggest just re-phrasing the sentence. – hunter2 Jul 5 '13 at 6:58
up vote 4 down vote accepted

In the first sentence, you are talking about two things, the number of students and the number of teachers. Let's call them A and B as a shorthand.

  • A and B is decreasing
  • A and B are decreasing

The second form is obviously correct:

The number of students and the number of teachers are decreasing day by day in the school.

In your second sentence, you are again talking of two things, and choosing between them:

  • Either A or B is decreasing
  • Either A or B are decreasing

In this case the first is correct. A potential difficulty arises because both A and B refer to a number of particular people which could be construed as a plural. So either is potentially correct. Most people would match the verb to the word immediately preceding it and use "students are".

Either a number of teachers or a number of students is participating in the college's festival.
Either a number of teachers or a number of students are participating in the college's festival.

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Not really. If you're just eliding "A [is decreasing] and..." then is would be correct. When your subjects are as short as 'A' and 'B' then you're probably not eliding but for the first example sentence, I'd say that's the most natural interpretation. – Jez Jul 4 '13 at 9:36
The sentence as given does not elide. It specifically mentions two numbers. If you want to change the sentence to "the number of students and teachers" then that's changing it. – Andrew Leach Jul 4 '13 at 9:37
How do you know it doesn't elide? The elided words wouldn't appear. It's a matter of how you interpret it. – Jez Jul 4 '13 at 9:38
What? "The number of students and the number of teachers [verb]" is not eliding anything. – Andrew Leach Jul 4 '13 at 9:39
Both are and is are acceptable (not is acceptable!) depending on whether you are talking of the number as a single entity or a group of individuals. – Andrew Leach Jul 4 '13 at 10:54

You simply need to decide whether “a number of people” should itself be construed to be singular or plural. Compare these two example sentences to find the right answer:

  • A number of people has come to the same realization.
  • A number of people have come to the same realization.

Which one of those is right? It is clear that only have works there, not *has. The explanation is more complicated, though.

The reason for this is that “a number of” functions as a pre-modifier to the noun phrase, just as “a lot of” does. That means it works like an adjective, and it therefore cannot change the grammatical number of the subject in a way that affects verb agreement.

Therefore in all cases you have given, the subject of your sentences is plural.

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This explanation applies only to the second example in the question. In the first example, the number itself is a subject of the verb, not merely a modifiers. The plural in the first sentences comes because there are two numbers, as explained in other answers. – Ben Voigt Aug 19 '14 at 4:47

Is the subject singular or plural? In the first example, the subject of the sentence is "the number", which is singular; "the number [...] is decreasing". In the second, the subject is "a number of...", which is plural; "a number of [x] are participating".

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I've seen people using are in the first sentence. This is referring to the number (ie as a concept of the numeric system), but there are two of these numbers under discussion, so are is the only correct option. – Sudhir Jul 4 '13 at 9:22
I don't agree; that example sentence merely elides "is decreasing" for the first "number". To pluralize it, you'd need to say something like "The numbers of students and teachers are decreasing". – Jez Jul 4 '13 at 9:34

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