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.
Possible answers:
a)is
b)are
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.
Best Answer
I believe your confusion is about what is the subject.
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.
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)