"I did" (past tense) puts the emphasis on the action of 'doing' itself in the past "I have done" (present perfect tense) puts the emphasis on the duration or the outcome of the action (which continues and is able to be noticed even at the present moment) * I washed my car. [<- you simply stated that you did a certain action in the past. and it has finished by now already] * I have washed my car. [<- here it implies that you have JUST finished washing your car and the result of it (a washed car) is obvious now] another pair of examples: * I didn't eat. (there's not enough information about whether or not you started eating.) (= Maybe you didn't eat yesterday, but you ate again this morning) * I haven't eaten. (you haven't eaten from the past till this very moment (may be extended to the future as well)) (= You didn't eat yesterday. You didn't eat this morning. Now it's 12 at noon you still haven't eaten anything.)

"I have done" refers to some unspecified time before now. "I did" would be used in the past tense, often with some sort of time specified. "I've visited China." [Correct] "I've visited China in 2004." [Incorrect] "I visited China in 2004." [Correct]