Subject-verb agreement

The grammatical subject of the sentence determines the "number" of the verb:
          • if the subject is singular, use a singular verb
An example:
          There is just one reason for the policy change.

          • if the subject is plural, use a plural verb
An example:
           There are many reasons for the policy change

The noun that followed the verb in both examples determined the number of the verb. Those were easy, but there are some traps that result from something known as false attraction that get a lot of writers confused. False attraction happens when some words are written between the verb and the grammatical subject.
An example:
          • His collection of paintings is the subject of a recent book.

The word collection is the subject and is singular, therefore the verb must be the singular is, though the  temptation might be to use the plural are as the verb to agree with the plural paintings.