The number of books in economics that we need to buy (is/are) three

1) The number of books in economics that we need to buy (is/are) three.

or

2) A number of books in economics that we can buy for the course (is/are) available in the college book store.

My approach:

I am confused here why is and are are used in the 1 and 2 sentence.

What they exactly mean according to subject verb agreement rules.

The number of books in economics that we need to buy (is/are) three

In this sentence,of books in economics that we need to buy is merely a modifier, the real subject is the number, which is always singular.

A number of books in economics that we can buy for the course (is/are) available in the college book store.

In this case, "a number of" is modifier, not unlike "a lot of", "some". So "books" are plural. So basically, "a number of " is an indiomatic phrase, should be treated as one organic compound word (adj) while "the number of " is basically three ordinary words. Admittedly I don't think people use the number of ...very often. It is just a grammar point teachers like to use. (I believe it is "a number of" instead of "the number of". Otherwise it won't make much sense)

• Can you teach me how to identify these kind of tricky sentences? Is there a trick behind this that makes you know? Sep 20, 2015 at 15:19
• Hi, I am not sure if there is any trick. It's just something to memorise, I guess. Sep 20, 2015 at 15:39
• So, what's the difference between "the number of" and "a number of" when determining whether it's a plural or singular? And, I'd like to see clearer explanation how "a number of" is a modifier and "the number of" is not. Sep 20, 2015 at 20:21
• @VictorBazarov "A number of" is a set phrase meaning "Several". oxforddictionaries.com/definition/english/number #2.1. If you remove all the subordinate clauses, the sentences are "The number [singular] is three", and "Several books [plural] are available". Sep 20, 2015 at 21:25

Even a native speaker of U.S. English (and I suspect some other varieties, though I can't speak for those) might have some difficulty with these two sentences.

In English class for native speakers, I recall it being said that if the subject of a sentence is a phrase such as "a/the number of books", then the noun number is the actual subject, that is, the subject is singular and the verb would be is rather than are.

But I think it would be very unusual for a native speaker to say, "A number of books is available in the store." One would more likely say, "A number of books are available in the store." In other words, "a number of books" is treated idiomatically as if it were the phrase "some books".

The first sentence speaks of a very specific number of books; namely, it says that the number is three. In that case, a native speaker would be likely to view the singular word "number" as the subject of the sentence, and to say "is" in the first example. But to be honest, I suspect that many native speakers would still say "are".

So if I were writing these sentences in real life, as an educated native speaker of U.S. English I would probably write:

1) The number of books in economics that we need to buy is three.

2) A number of books in economics that we can buy for the course are available in the college book store.