edgood  —  Grammar Tips
The grammatical subject of a sentence or clause is a noun, a group of words acting as a noun, or a pronoun. The subject names whatever is asserted by the verb. Usually, the subject precedes the verb, as in She walked to work. But when you ask a question, you put an auxiliary verb before the subject, as in Will she walk to work? Examples of subjects in the following are underlined:
The committee wrote the manual. (single noun acting as the subject)

She won the race. (pronoun acting as the subject)

To win the race became his passion. (infinitive phrase acting as the subject)

Achieving success remained his principal goal. (gerundive phrase acting as the subject)

That she won the race didn't surprise us. (nominal clause acting as the subject)
Only the grammatical subject of a clause determines the number of the verb. If the subject is singular, the verb must be singular. If the subject is plural, the verb must be plural.

The predicate noun of a sentence never determines the number of the verb. Thus, the following is correct:
Our biggest problem is the thousands of cars clogging the streets.