You have to choose the correct one. For example: un- can be added to 'happy' to make 'unhappy', but dis- or mis- couldn't. Adding in- can be particularly tricky. There are three rules to remember. 1.

These words are going to have the prefixes dis, un and in, and remember, they're going to mean not or the opposite of, when we add it to that word. Let's get started. All right.