I recently discovered that cowardly, which looks like an adverb, is actually also an adjective. So far so good. Then what is the difference between cowardly and coward, and is there any preferential usage of either?
The difference is that cowardly is usually an adjective, while coward is almost always a noun. Hence:
That dog is a coward.
That cowardly dog runs at the sound of hooves.
In the rare cases that coward is an adjective, it is always prepositive (directly before the noun):
That coward dog just turned tail.
That dog is coward. (Coward is not prepositive - before the noun - hence this is wrong)
As to meaning differences (I've been dealing with usage), you would use cowardly if you weren't necessarily name-calling or venting; the term sounds more objective. You could quietly use it, without strong feelings. Coward, on the other hand, always connotes strong feelings. If you use coward as an adjective, I'm more likely to think you are name-calling or venting.
The "also" in the first sentence of the question suggests that "cowardly" can be an adverb as well as an adjective. I don't think this is true, or at least I can't think of an example. You can't properly say a guy fought cowardly. You'd say he fought like a coward. I'd actually like to use it, or any simple adverb, to mean "like a coward." (That's why I clicked on this post.) What are the alternatives? "Uncourageously" is too weak. "Pussillanimously" is the best I can come up with, but it would be weird if such a "big" word were the only adverb for such a basic concept.