Group words, or collective nouns, are followed by singular verbs when we are talking of the group as a whole. They take a plural verb when we are talking about the individual members of the group.

Compare:

The jury is in the court room. (Here we use a singular verb because we are talking of the group as a whole.)
The jury are still debating the case. (Here we use a plural verb because it is the individual members of the group who are debating the case.)

Note that in American English collective nouns are always followed by singular verbs.

Nouns that always need a plural verb

Some nouns are always followed by plural verbs. Examples are: binoculars, scissors, spectacles, glasses, knickers, pyjamas, tongs, shears, trousers etc. Note that it is wrong to say ‘a binocular’ or ‘a spectacle’. If the singular aspect is to be expressed use the expression ‘a pair of’. Examples are: a pair of trousers, a pair of tongs etc.

The binoculars are very useful at the races.
His new glasses were on the table.
The tongs were by the fireplace.
The garden shears are used to clip the hedges.

Note that we use a singular verb when the phrase ‘a pair of’ comes before the noun.

Compare:

Bring me the knickers that are on the line.
Bring me a pair of knickers that is on the line.
Pyjamas are in the drawer.
A pair of pyjamas is in the drawer.

Cases where a singular verb is necessary

When a singular noun and a plural noun are connected by or, the verb agrees with the nearest noun.

Grapes or a melon is suitable. (Here we use a singular verb because the nearest noun (melon) is singular in number.)
A melon or grapes are necessary. (Here we use a plural verb because the nearest noun (grapes) is plural in number.)

When two singular nouns are joined by or, a singular verb is used.

Jam or butter was not available in the store.

Note that it would be better to use the plural noun second; then a plural verb is necessary.

A singular noun is used after ‘one of’, ‘none of’, ‘each of’ and similar expressions.

One of the calves is sick. (NOT One of the calves are sick.)
One of our students has won a prize.