hi Diamond, just to let you know that the word "coward" is a noun and "cowardly" is an adjective, and to answer your question if you can use them interchangeably, the answer is no. In this sentence " I'm too cowardly to tell her anything." the person is trying to describe himself/herself here as "cowardly" so it is only proper to use this word as an adjective. I hope i have helped you in some way.