A singular antecedent needs a singular pronoun, and a plural antecedent needs a plural pronoun. For example, “Bill took his hat” and “The girls read their book” are correct.
A phrase after the noun does not change the plurality of its referent. For example, “the bowl” and “the bowl of marbles” will have the same pronoun, such as “The bowl sits in its drawer” and “The bowl of marbles sits in its drawer”.
When you have indefinite pronouns as the noun in a sentence, the referent must still agree in plurality with the indefinite pronoun acting as the antecedent. Singular indefinite pronouns are each, either, neither, one, no one, nobody, nothing, anyone, anybody, anything, someone, somebody, something, everyone, everybody, and everything. These singular indefinite pronouns require a singular referent. For example: “No one can lick his or her elbow”. Plural indefinite pronouns are several, few, both, and many. These indefinite pronouns require a plural referent, such as “Several of the children play with their toys”. The plurality of an indefinite pronoun can be modified by a prepositional phrase. When the object of the preposition is countable, such as “eggs” or “chairs”, use a plural referent. For example, “All of the fries have lost their heat”. When the object of the preposition is uncountable, such as “water” or “flour”, use a singular referent. For example, “All of the water spilled out of its cup”.
When there are multiple subjects in a sentence joined by “and”, always use a plural referent. For example, “Ben and Samantha did their best”.
When there are multiple subject joined by “or” or “nor”, the referent must agree with the subject closer to the pronoun. For example, “Neither the girl nor the boys would clean their hands”.
Collective nouns, or nouns that represent groups such as team, pack or class, can be plural or singular depending on whether the group is acting as a unit or as individuals. For example, “The class gave its opinion” versus “The class members gave their opinions”.
Title of things, such as organizations, movies, and countries, require singular referents. For example, “The United States has 50 states within its borders”.
If you have “every” or “many a” before a noun or a series of nouns, the referent must be singular, such as “Many a dog has had fleas in its coat”.
Plural subjects with a singular meaning, such as news or measles, need a singular referent. “The news cares more about its rating than current events” is correct.
“A number of” is plural, while, “The number of” is singular. “A number of people love their mother” is correct, as is, “The number of stray cats has increased its ranks”.