"You shouldn't use "a" with a plural"
Since I'm not the author of the article, you should've said:
"One shouldn't use 'a' with a plural"
Sorry, couldn't resist ;-)
I was indirectly addressing the author through you;-)
I often send this test to folks in need of a grammar wake up call.
http://www.okcupid.com/tests/take?testid=14457200288064322170