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marked as duplicate by FumbleFingers, Carlo_R., tchrist, RegDwigнt Jan 16 '13 at 9:43

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Either is possible. It depends on the context. – Peter Shor Jan 16 '13 at 5:03
Visit and join ESL area51.stackexchange.com/proposals/41665/… – Kris Jan 16 '13 at 6:56
Please have a look at the linked questions. The first one provides a general overview of how the different tenses in English correspond to one another. The second one, and the related questions linked from there, address your question more directly. Lastly, the third one addresses an important difference between British and American English. – RegDwigнt Jan 16 '13 at 9:46

The perfect tense, used in the second sentence, indicates something done in the past, which has a continuing effect on the present. To ask the first could mean nothing, as their hands could have been washed yesterday, or maybe even today and are now dirty. The second, however, is much more effective in interrogation.

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