In traditional grammar, the subject of a sentence is defined as what the sentence is about. The predicate, or the remainder of the sentence which contains the verb, describes action taken by the subject. In the sentence "The policeman arrests the suspect," "policeman" is the subject. The verb, "arrests," agrees with "policeman" because both are third-person singular.
Words describing multiples are called collective nouns. Conventional grammar demands that they take a singular verb, as in "The police force IS holding its annual meeting." However, some grammarians argue that this rule fails to take context and point of view into consideration. Someone reading about such a police meeting, for instance, might see "the police force" as a single entity, while someone attending the meeting might focus on the members and thus see "the police force" as a collective plural. Thus, according to some grammarians, "The police force ARE having their annual meeting" could be correct.
"Its," "their," "his" and "her" are possessive pronouns. The English language lacks a gender-neutral pronoun to refer to people. Traditionally, the generic masculine was used to refer to any group that had even one male in it -- even if the group also included females. Thus, "Any member of the police force with long hair must wear it under HIS hat." Substituting "his or her" answers the question of gender yet undermines readability. Dennis Baron, Professor of English and Linguistics at the University of Illinois, points out that in the 1800s, grammarians floated the idea of the hybrid pronoun "hiser" to replace "his or her" but it never caught on. A case can and has been made for the use of "their" as a singular gender-neutral pronoun ("Any member of the police force with long hair must wear it under THEIR hat.") but its level of acceptability depends on the user and audience. The debate continues.
William Safire, author of the book "How Not To Write," was once asked to adjudicate a skirmish between riled grammarians and McCall's magazine. The magazine had run advertisements featuring photographs of famous beauties, each with the ironic caption: "One of the drab homebodies who reads McCall's." Safire had to decide whether "reads," which referred to "one," should have used the plural form "read" to refer to "homebodies." In his deliberations, Safire chose to disregard the rule specifying that a verb must agree in number with the last noun of a compound subject. He concentrated instead on the "essence" of the meaning but in the end, his verdict upheld the rule. "Homebodies" carried more weight than "one" in this context, he decided, so the verb should be "read." McCall's changed its captions.