Don't ask me -- I suck at remembering which part of speech is called what (especially since there's often a lot of fluidity in which one you use). As a native speaker of English, however, I know which word is the right one to use, because it's the one that sounds right when you say it out loud.

Frankly, I have no idea how people manage to learn English. Other languages are basically neat and orderly -- English is a disaster all around.