A singular subject requires a singular verb form, whereas a plural subject requires the plural verb form.

The three common verb pairs that cause the most problems are is/are, has/have and do/does.

The subject of the above sentence is three common verb pairs. Since pairs is plural, we need to use the plural form of the verb, which in this case is are.

The BBC has a legitimate interest in the rules and regulations passed by the Eurozone.

The phrase "Rules and Regulations" is indeed plural, but the subject of the above sentence is The BBC. Note that The BBC is used as a third person noun (It is an It).

Billy and Joel do the networking for our webserver in Oakland.

Here is another instance of third person nouns being the subject of the verb. As the subject is plural (two third person nouns connected by and) so too should their verb be the plural do.

It can seem counter-intuitive for words ending in -s to be singular, but such is the case for the verbs has and does. Compare/Contrast not just how the verb changes, but also how other words must change with them:

  • Billy is a good friend.
  • Billy and Jane are good friends.

  • Billy has a nice car.

  • Billy and Jane have nice cars.

  • Billy does what he wants.

  • Billy and Jane do what they want.