 As previously mentioned, the usage for the past simple tense is when you want to talk about completed actions in the past at a definite time frame, so we have sentences like Mark worked last night. He worked in the past, when was it? Last night. We can also ask questions. When did Mark work? Although the time isn't mentioned in the question, what we're implying is the time will be mentioned in the answer. And by saying when did Mark work, we're indicating that it is in the past rather than saying when does Mark work, which would bring us back up into the present.