A man and a woman have been here (not has been here).
Boxes are scarce (not is scarce).
When were the brothers here (not when was)?
62. Agreement of Subject and Verb in Number. The general rule to be borne in mind in regard to number, is that it is the meaning and not the form of the subject that determines whether to use the singular or the plural form of the verb. This rule also applies to the use of singular or plural pronouns.
Many nouns plural in form are singular in meaning; as, politics, measles, news, etc.
Many, also, are treated as plurals, though in meaning they are singular; as, forceps, tongs, trousers.
Some nouns, singular in form, are, according to the sense in which, they are used, either singular or plural in meaning; as, committee, family, pair, jury, assembly, means. The following sentences are all correct: The assembly has closed its meeting, The assembly are all total abstainers, The whole family is a famous one, The whole family are sick.
In the use of the adjective pronouns, some, each, etc., the noun is often omitted. When this is done, error is often made by using the wrong number of the verb. Each, either, neither, this, that, and one, when used alone as subjects, require singular verbs. All, those, these, few, many, always require plural verbs. Any, none, and some may take either singular or plural verbs. In most of these cases, as is true throughout the subject of agreement in number, reason will determine the form to be used.
Some nouns in a plural form express quantity rather than number. When quantity is plainly intended the singular verb should be used. Examine the following sentences; each is correct: Three drops of medicine is a dose, Ten thousand tons of coal was purchased by the firm, Two hundred dollars was the amount of the collection, Two hundred silver dollars were in the collection.