The prefix in- attaches to the beginning of words and usually means “not” or “into.”

When in- means “not,” it creates opposites: accurate becomes inaccurate, visible becomes invisible, and complete becomes incomplete. A student giving an incorrect answer isn't giving the right one. An inappropriate joke isn't suitable for the situation. Notice that before certain letters, in- changes form to make pronunciation smoother: it becomes im- before m or p (impossible, impolite), il- before l (illegal, illogical), and ir- before r (irregular, irresponsible).

When in- means “into” or “within,” it works differently. To include means to bring something in. To inhale means to breathe in. An inmate lives inside a prison. When you look inward, you examine your own thoughts and feelings inside yourself.

The tricky part? Sometimes the same word can be confusing. Inflammable actually means able to catch fire easily (from the “into flame” meaning), even though it looks like it should mean “not flammable.” That's why many warning labels now use flammable instead to avoid dangerous confusion.