Can some words serve as both adjective or adverb? [duplicate]
In the following sentences:
- I can move quicker than she can.
- She moves quickly, but I can move quicker still.
- Between us, I move quickest.
- Between us, I move quicker.
- I am even quicker than she.
I am apparently trying to modify the verb "move" with the adjectives "quicker" and "quickest".
It seems to me that I have been speaking like this all my life, but just now I've been told that adjectives may not modify verbs, and that these sentences are grammatical garbage. Yet, when it comes to:
I can move quicker than she can.
vs
I can move more quickly than she can.
The first sentence sounds just as good as the second, to me, at least. Cannot the word "quicker" serve as both adjective and adverb?
The issue here is that some adjectives can be used as adverbs to modify verbs. While most adjectives get -ly when turned into adverbs, a few do not. And in some cases they can be used without -ly but only in a somewhat less formal context. I should say quick (and quicker) can be used as adverbs informally, but perhaps not in formal prose. The adjective fast, however, can be used as an adverb even in (fairly?) formal prose:
Germany moved fast and secured the fortress before the French could intervene.