A pair of trousers

Phrases that combine a pair of and binary nouns such as trousers and binoculars are one of those problematic cases of subject-verb agreement.

One reason is that both pair and trousers refer to things comprising two parts, making them potentially troublesome words. To compound the problem, words such as trousers and scissors end with the plural suffix "s", with the consequence that the proximity rule may be wrongly applied.

Not surprisingly, you sometimes see a plural verb used:

?A pair of glasses were broken.

The correct verb in the above is the singular was, since it agrees in number with the singular noun (or partitive) pair.

The rules, as prescribed by most grammar books, are fairly straightforward:

Rule 1: If the noun pair is present, use a singular verb:

(1) The pair of trousers belongs to Jane.
(2) A pair of scissors was found.

Rule 2: If more than a pair is referred to, use a plural verb:

(3) Several pairs of shoes are missing.
(4) Two pairs of spectacles were found at the pool.

Rule 3: If pair is absent, use a plural verb:

(5) These scissors are blunt.
(6)  Those shoes don't belong to him.