Sometimes a singular verb is needed even though the subject is a plural noun. Study the following sentences.

Two years was a long time to be away. (More natural than ‘Two years were a long time to be away.’ Here the verb agrees with the phrase ‘a long time’.)
Fifty dollars is too much to lose. (NOT Fifty dollars are too much to lose.)

When a singular noun and a plural noun are joined by or, the verb agrees with the nearest noun. Note that in this case it would be better to use the plural noun second; then a plural verb would be used.

A bomb or bullets are not allowed on the plane. OR Bullets or a bomb is not allowed on the plane.

After one of, we use a plural noun and a singular verb.

One of my cousins is a doctor. (NOT One of my cousins are doctors.)

Some nouns always take a plural verb. Examples are: glasses, binoculars, scissors, knickers, pajamas, tongs, shears, tweezers etc.

‘Where are my glasses?’ ‘They are on your nose.’ (NOT Where is my glasses?)

To indicate the singular aspect of the word, we can use the expression ‘a pair of’. In that case a singular verb is used.

Bring me the pair of knickers that is hanging in the cupboard. (NOT Bring me the pair of knickers that are hanging in the cupboard.)
Tweezers are useful when handling stamps. (NOT Tweezers is useful when handling stamps.) (NOT A tweezer is useful when handling stamps.)