Question: I have a question on indefinite articles.
I thought **a** is only used with singular nouns, by definition.

Why is "in a couple weeks" the right way? Isn't a couple weeks more than one?
I thought it would be "in couple weeks" but I see everywhere that it's used as "in a couple weeks". Can someone please be kind and explain why this is so?

Answer: 
In the **idiom** "*a couple weeks*" the indefinite article "*a*" modifies the singular noun "*couple*" according to the rule of indefinite articles.

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> Another example:
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> My wife and I are ***a*** *couple*.
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As @John Lawler so aptly pointed out, the confusion is in the missing (yet still implied) preposition "*of*" that creates the **illusion** that "*a*" is modifying the plural "weeks". That would leave "*couple*" as an orphan in the phrase. *Couple* is not generally used as an adjective, and it couldn't correctly be used as an adjective in this case.

Idiom tends to defy grammar, but there is often a correct grammatical form supporting the idiom in its etymology.
"A couple weeks" does indeed sound incorrect because it should correctly be written as "a couple **of** weeks". It makes perfect sense if it is written or spoken correctly because in this case the "couple of weeks" becomes a singular group of weeks consisting of two weeks. It's like saying "a flock of geese" which is a singular flock consisting of many geese, or similarly "a pair of socks" which is a single pair of socks.

If said incorrectly missing the "of" the way many Americans do the whole sentence sounds odd.