a pair of

грам.

a pair of + V — pl or sing?

Since a "pair of trousers" is a unit (one article of clothing), I would say "a pair of trousers is on the sofa".

"A pair of trousers is on the sofa." But "your trousers are on the sofa." A bit unusual, but saying "a pair of" stresses the unity, but the very same garment becomes, somehow, plural without the phrase. Same thing with "a pair of glasses is on the desk" and "my glasses are on the windowsill." And, just to confuse things a bit, I don't believe scissors follows the same path. "A pair of scissors is on the desk" and "the scissors is on the table." Or does someone out there allow "the scissors are on the table"? Another opinion from a U.S. speaker: I allow "The scissors is on the table" and "The scissors are on the table." One thing that might be confusing in learning English is that when a number and a unit of measure join to form an adjectival compound, the word for the unit of measure is singular:

— a five-pound bag of sugar (not a five-pounds bag of sugar)

— a two-mile hike

— a three-day weekend

— a hundred-dollar gift certificate

— a twenty percent increase

"Perhaps the stranger had given him a spare suit of his own, for the jacket sleeves were too short and the trousers were baggy." [translated by Helen Saltz Jacobson, 1977; Macmillan Publishing Co., Inc, New York; Collier Macmillan Publishing, London]