The basic rule is that a singular verb (e.g. is, does, has) is used with a singular subject and that a plural verb (e.g. are, do, have) is used with a plural subject. For example,
The girl is going to a party.
The girls are going to a party.
One of the most common mistakes made when trying to make subjects and verbs agree occurs when the word of gets involved. For example,
Their selection of cheeses was (not were) perfect.
Care should be taken when the subject of a sentence contains two nouns or pronouns. As the examples show, it is possible to use the singular instead of the plural verb:
When the subject of a sentence contains two singular nouns connected with and, use a plural verb. If they are connected by or, use a singular verb. For example,
Jack and Sam are (not is) at home.
Jack or Sam is (not are) at home.
There are a few exceptions. In the example below, Health and Safety is a compound noun.
Health and Safety is important.
When the subject contains one plural and one singular noun or pronoun use the verb that matches the noun or pronoun that is nearest the verb. For example,
Jack or his friends sing at the Karaoke club.
The boys or Jack sings at the Karaoke club.
Make sure to correctly identify the subject. It is easy to mistake a singular for a plural subject. For example,
The long list of rules are hard to read.
The long list of rules is hard to read.
Watch for nouns and pronouns within dependent clauses or phrases that interrupt independent ones. For example,
The lead singer, as well as the two guitarists, was (not were) having a particularly good night.
The website, including all the main pages, is (not are) hard to navigate.
Use a singular verb with each or either. For example,
Each of these drinks is cold and refreshing.
If you are wondering whether to wear a jacket or a suit, either is acceptable.
The same goes for everybody and anybody. For example,
We are very accommodating, everybody (or anybody) is welcome.
Collective nouns, even though they suggest more than one, can be either singular or plural depending on what they refer to. In the examples below the collective noun refers to a single, impersonal unit that can be thought of as all doing the same thing.
The team is delighted with the result.
My family has always supported me.
The entire class was off sick.
But sometimes they are plural as in the examples below, where the group are not being referred to as one single unit, and are acting (or feeling, or being) as a group of individuals.
The team are changing into their uniforms.
The family are singing different songs at the party.
The class were doing their exams.
Note: Usually, when the collective noun is doing things that people do (e.g. drinking, working, talking, eating, feeling etc.), a plural verb is used.
Currency is a special case when it comes to subject/ verb agreement. When pounds are used to describe an amount of money, pounds take a singular verb. When used otherwise, a plural verb is required. For example,
Twenty pounds is a lot of money to many people.
You get a poor rate when pounds are exchanged at the airport.