A prefix is a group of letters attached to the beginning of a word to change its meaning. The word prefix itself shows you how it works: pre- means “before,” and fix means “attach,” so a prefix is something attached before.

When you add the prefix un- to happy, you get unhappy, which means the opposite. Add re- to write, and you get rewrite, meaning to write again. The prefix mis- in misunderstand tells you someone understood something incorrectly.

Prefixes are powerful tools because once you know what they mean, you can figure out hundreds of words you've never seen before. If you know pre- means “before,” you can understand preview (seeing before), preheat (heating before), and prehistoric (before recorded history). If you know dis- means “not” or “opposite,” you can decode disagree, dislike, and dishonest.

Learning common prefixes like anti- (against), sub- (under), super- (above), and inter- (between) is like getting a key that unlocks the meaning of countless words. When you encounter an unfamiliar word like antifreeze or submarine, you can use your knowledge of prefixes to figure out what it probably means.