Some nouns, like committee, clergy, enemy, group, family, and team, refer to a group but are singular in form. These nouns are called collective nouns. In American usage, a collective noun takes a singular verb when it refers to the collection considered as a whole, as in The family was united on this question or The enemy is suing for peace. It takes a plural verb when it refers to the members of the group considered as individuals, as in My family are always fighting among themselves or The enemy were showing up in groups of three or four to turn in their weapons. In British usage, collective nouns are more often treated as plurals: The government have not announced a new policy. The team are playing in the test matches next week.
1. Be careful not to treat a collective noun as both singular and plural in the same construction. Thus you should say The family is determined to press its (not their) claim.
2. Collective nouns always refer to living creatures. Similar inanimate nouns, such as furniture and luggage, differ in that they cannot be counted individually. That is why you cannot buy a furniture or a luggage. These nouns are usually called mass nouns or noncount nouns. They always take a singular verb: The bedroom furniture was on sale.