Nouns used only in the plural
Some English nouns are usually used only in the plural. Such nouns take a plural verb, and generally have a plural form. For instance, the following nouns, which all refer to objects with two parts, are usually used only in the plural:
e.g. jeans
pajamas
pliers
scissors
If it is desired to refer to such objects individually, the expression pair of is often used.
e.g. a pair of jeans
a pair of pajamas
a pair of pliers
a pair of scissors
When the expression pair of is used as the subject of the verb, the verb must agree with the word pair.
e.g. Jeans are fashionable.
A pair of jeans is expensive.
Pliers are very useful.
A pair of pliers is often useful.
In the above examples, the nouns jeans and pliers take the plural verb are, and the noun pair takes the singular verb is.