When do you use didn't or doesn't?

Answer 1

When you use didn't:

Didn't can be used in two ways, the way of asking a question "Didn't you get the dog food?" and in the way of saying someone did not do something "You didn't let me in the door!"

When you use doesn't:

In a question "Doesn't the car need washing?" and in something or someone or anything doesn't do something "He doesn't let me play checkers with him!"

Answer 2

"Didn't" is the past tense form of "do not" or "does not," used for actions that happened in the past. "Doesn't" is the present tense form of "do not" or "does not," used for actions happening in the present or habitual actions.