Some nouns, like "underpants", only have a plural form and cannot be used with numbers or the article "a".
It's "a pair of underpants". In American English, "underpants" is a plural noun that refers to both women's and men's underwear. In British English, this term refers to only men's underwear.
He's only wearing a pair of underpants, nothing else.
He's only wearing an underpants, nothing else.
Some nouns, like "underpants", only have a plural form and cannot be used with numbers or the article a/an, but you can use the determiners "a pair of", "two pairs of", "three pairs of", etc. to count these items.
John has twenty pairs of underpants.
In everyday English, however, we commonly use a possessive pronoun, such as my, your, his, her, etc.
Are your underpants wet?
But use no determiner to talk about underpants in a general way.
Underpants are typically covered with other clothes.
Be aware that plural nouns take a plural form of a verb, so we say "underpants are" (not "underpants is").
Are your underpants clean?
Is your underpants clean?
Follow the same convention with other nouns that only have a plural form, including: