Plural nouns, like "trousers", cannot be used with numbers or the article "a". However, you can use the expression "a pair of".
It's "a pair of trousers". In British English, we can use the plural noun "trousers" to refer to a piece of clothing for the legs used by men and women ("pants" in American English).
We bought a pair of trousers last week.
We bought a trousers last week.
Plural nouns, like "trousers", cannot be used with numbers or the article "a". However, you can count these articles of clothing by using the expressions "a pair of trousers", "two pairs of trousers", "three pairs of trousers", etc.
There are two pairs of trousers in the closet.
We can also use a possessive pronoun (my, your, his, her, etc.)
Your trousers are too loose.
But use no determiner to speak about trousers in a general way.
Trousers can be made from polyester, cotton, denim, nylon, etc.
And remember, plural nouns require a plural form of a verb, so we say "trousers are" (not "trousers is").
Are your trousers comfortable?
Is your trousers comfortable?
Follow the same strategy with other nouns that only have a plural form, including: