Rule: Don’t use indefinite articles with plural words.
Incorrect: He has a muscular arms.
Correct: He has muscular arms.
Rule: Use a partitive structure with words that are never singular.
Incorrect: She wore a blue jeans. (Other examples: shorts, pants, jeans, leggings)
Correct: She wore a pair of blue jeans.
Exception: Words like “series” and “species” look plural, but they are also singular and can be used with an article.
Correct: I found a series of books at the bookstore that I really enjoy reading.
Correct: The Golden Palm Weaver is a species of bird in the Ploceidae family.
Exceptions that defy explanation: An amazing two days, an awkward ten minutes, a whopping twenty-five pounds, a good many books, etc.