In English, finite verbs are decided by the number of the noun used in the place of the subject. The other verb forms are syntactically aligned to this symmetry or concord. In everyday language, this harmony is termed as subject-verb agreement.
E.g. the subjects like Alice, he, tree, etc. are singular and will take singular verbs like is, has, falls, etc.
To ensure agreement between the verb and the subject, certain rules are to be followed. Some of the rules are mentioned below.
Two or more singular nouns or pronouns joined by ‘and’ require a plural verb.
For example: The girl and her brother were invited for the party.
Two or more singular nouns or pronouns joined together by ‘or’ or ‘nor’ will take a singular verb.
When one of the nouns joined by ‘or’ or ‘nor’ is plural, the verb must be plural and the plural subject is to be placed near the verb. But, when the subjects joined by ‘or’ or ‘nor’ are of different persons, the verb agrees with the noun which stands nearer to it.
For e.g.: Either you or some members of your team have played foul.
Neither you nor I am talking to Veronica.
Nouns plural in form and singular in meaning take singular verb
Collective noun usually takes singular verb but, when it refers to its constituents, the verb will be plural.
For e.g. the bunch of keys is missing.
The class were enthusiastic with their inputs.
Error of proximity: Sometimes we will erroneously make the verb to agree with a noun other than its proper subject because it stands near the said verb. This is called error of proximity.