Does is the third-person singular present form of the verb “do.” When you talk about what someone or something is doing right now, habitually, or generally, you use does instead of do if your subject is he, she, it, or any singular noun.

For example: “She does her homework every afternoon” or “The machine does exactly what we programmed it to do.” You wouldn't say “she do” or “it do.” The word does makes the sentence grammatically correct when talking about one person or thing.

You also use does to form questions and emphatic statements: “Does the library open at nine?” or “He does try hard, even when things are difficult.” In questions, does comes before the subject.

Notice that does sounds like “duz,” not like “doze” (which means to sleep lightly). This trips up many people in writing, since the spelling doesn't obviously match the pronunciation.

The word can feel invisible because it's so common, but using does correctly helps your meaning stay clear. “The teacher does grade fairly” is easy to understand. “The teacher do grade fairly” can distract listeners from what you're actually trying to say.