Today I was diagramming a sentence when I noticed something that confused me. I had a sentence that was basically like this:
A parent's greatest concern is rearing his children correctly.
"Concern" is obviously the noun there, with "parent's" and "greatest" as adjectives modifying it, but what is "a"? It's an article which I thought were always adjectives, but it looks like it's modifying "parent's", which would have to make it an adverb since you can't stack adjectives, right?
What is it? Can articles be adverbs if they're modifying possessive adjectives? Do you just stack adjectives for some reason? Does it somehow modify the subject "concern" for some reason?