Doesn't is a contraction of “does not,” used to say that something is not happening, not true, or not the case. When you say “She doesn't like broccoli,” you're stating that she does not like broccoli. When you observe that “This key doesn't fit the lock,” you mean it does not fit.

English speakers use doesn't with he, she, it, or any singular noun. You would say “The dog doesn't bark much” or “My brother doesn't play the piano.” For I, you, we, or they, use don't instead: “I don't understand” or “They don't agree.”

The apostrophe shows where letters have been removed. In casual speech and informal writing, doesn't sounds more natural than the formal “does not.” However, when you want to add emphasis or formality, you might say both words: “He does not have permission” sounds more serious than “He doesn't have permission.”

Watch out for a common mistake: some people incorrectly say “don't” with singular subjects, as in “He don't know.” The correct form is “He doesn't know.” Getting this right helps your speech and writing sound clear and confident.