Reviewing the original question: There is no English grammar rule for forming the opposite of a word. This is an idiomatic usage, and you must memorize them all. Buhahaha.
You would be sitting pretty, however, if you wished to consider Esperanto. Although some opposites come in pairs (varma/frida; mola/dura) you can make any antonym by adding mal- (varma/malvarma, mola/malmola). So nyah.