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Prefixes are a group of letters that change the meaning of a word when they are added to the start. Most prefixes mean a similar thing when they're added to different words.

1. un usually means not. For example, unhappy, unlocked, unfair
2. dis and mis usually have negative meanings. For example, disagree, disobey, misbehave, mislead
3. re usually means again or back. For example, redo, reappear, redecorate
4. sub usually means under. For example, subheading or submarine