Other forms: forewarned; forewarning; forewarns
When you forewarn your friend about the pop quiz in math, you let her know beforehand that it's happening.
You should forewarn your friends about the highway construction that might slow them down when they come to visit you, and you could forewarn your shy exchange student that your dad likes to interrogate people at the dinner table. The verb forewarn combines fore, as in "before" or "early," with warn, "inform of danger or problems." You may have heard of the proverb "forewarned is forearmed," which means you're at an advantage if you know what's coming in advance.