Subjects and verbs must agree with one another in number (singular or plural). Thus, a singular subject (she, Marvin, Bike) takes a singular verb (is, goes, shines), whereas a plural subject takes a plural verb. This kind of agreement between the subject and the verb is called the Subject-Verb Agreement.
Nouns or pronouns joined by ‘and’ make a plural subject.
For example: The girl and her brother were invited for the party.
When a number of nouns refer to one single person or thing then the subject is singular.
For example: My friend, philosopher and guide advises me to be honest in my deeds.
If the noun or pronoun after “or”, “nor”, is singular then a singular verb will be used. If it is plural, then a plural verb will be used.
For example: Neither you nor I am supposed to go to that room.
The number of the subject does not change because of the qualifiers like with, as well as, besides, in addition to, together with, no less than.
For example: The lost bag, with its contents, was finally recovered.
When many a, each, every, everyone, anybody, either, neither, no one, nobody are used, the subject is singular.
For example: Either of the dresses is worth buying.
Sometimes the names of places such as the United States, books such as ‘The Three Musketeers’ or Newspapers such as ‘Times’ appear to be plural but they are always singular as they refer to one single entity.
For example: ‘Three Men in a Boat’ is written by Jerome K. Jerome.
When a plural noun refers to one amount or quantity or length it is singular.
For example: Thirty kilometers is a long distance.
There are many nouns which appear to be plural but are used singular.
For example: No news is good news.
Nouns that appear as plural, but used as singular
Innings, politics, physics, summons, mechanics, news, mathematics
There are nouns which are always plural.
For example: Kevin, please give me the scissors.
Collective nouns are used as singular as they refer to a group. But sometimes when they refer to the members of the group, they are used as plural (Noun of Multitude).
For example: The committee is planning to give some subsidy to the employees.