Did is the past tense of the verb do. When you say “I did my homework,” you're explaining that you completed it at some point before now. When your teacher asks “Who did the experiment correctly?” she wants to know which students successfully performed it in the past.

The word did shows up constantly in everyday speech. You use it to talk about completed actions: “She did well on the test,” “They did the dishes after dinner,” or “He did a cartwheel in gym class.” It's also essential for forming questions about the past: “Did you finish your book?” or “Did the package arrive?”

Sometimes did adds emphasis to a statement, especially when you want to insist something really happened. If your brother doubts you cleaned your room, you might say “I did clean it!” with extra stress on the word. This emphatic use makes your point stronger than simply saying “I cleaned it.”

Did is one of those small words that works incredibly hard in English. While it might seem simple, it's actually doing important grammatical work every time you use it to talk about anything that happened before this moment.