Since the word "pants" only has a plural form, you cannot use the article "a" or numbers with it. However, you can say "a pair of pants", "two pairs of pants", etc.
It's "a pair of pants". In American English, we use the plural noun "pants" to refer to the piece of clothing that covers the lower part of the body from the waist to the foot ("trousers" in British English).
He bought a pair of pants yesterday.
He bought a pants yesterday.
Since the word "pants" only has a plural form, you cannot use the article "a" or numbers with it. However, you can say "a pair of pants", "two pairs of pants", etc.
There are three pairs of pants on the bed.
In everyday English, however, we commonly use a possessive pronoun (my, your, his, etc.).
My pants are in the closet.
But omit the determiner to use "pants" in a general sense.
Leather pants can be casual.
Using a/the with articles of clothing
Be also aware that plural nouns, including "pants", always take a plural form of a verb; so we say "pants are/have", not "pants is/has".
Are your pants clean?
Is your pants clean?
Other pieces of clothing that do not take the article "a" can be found below:
