Didn't is the shortened form of “did not,” used to say that something in the past didn't happen or wasn't true. When you say “I didn't finish my homework,” you're explaining that finishing your homework is something that did not occur. When someone asks “Didn't you hear the bell?” they're questioning whether you heard it, expecting that maybe you did.

This contraction appears constantly in everyday speech and informal writing. You might say “She didn't call me back” or “They didn't win the championship” or “We didn't see that coming.” In each case, you're talking about something that could have happened but actually didn't.

In formal writing, people often spell out “did not” instead of using the contraction, but in regular conversation and casual writing, didn't feels natural and comfortable. Notice how didn't always refers to the past: for present situations, you'd say “don't” or “doesn't” instead.

The word also appears in questions, like “Didn't I tell you?” or “Didn't we agree on six o'clock?” These questions often suggest surprise that something wasn't done or wasn't remembered.