It's "a pair of jeans". Since "jeans" is always plural, you cannot use the article "a" with it.
It's "a pair of jeans". Since "jeans" is a plural noun, you cannot use the article "a" with it.
I have to buy a pair of jeans.
I have to buy a jeans.
Some nouns, like "jeans", only have a plural form, so we don't use “a” with them. However, you can use the determiner "a pair of", "two pairs of", "three pairs of", etc.
I have a new pair of jeans that I love but are too tight.
She has six pairs of jeans in the closet.
Alternatively, you can use other determiners for plural nouns, including "the", "some", or "any".
The jeans are great!
But use no determiner when talking about jeans in a general sense.
Jeans are typically made of cotton.
Be also aware that plural nouns take a plural form of a verb, so we say "jeans are" (not "jeans is").
My jeans are too short.
My jeans is too short.
Follow the same convention with these nouns: