They CAN be interchangeable in AmE, but not in all cases. In AmE, a past simple can be used in the place of a present perfect when you're talking about a finished action in an unfinished, unspecified or recent time period. This includes sentences with 'just', 'ever' 'yet' and 'already': "Have you eaten yet?" - Br E "Have you eaten yet?" or "Did you eat yet?" - AmE* This doesn't happen when the present perfect is used to describe a continuing state. For example, "He has lived here since he was a child" or "I've had this car for 10 years" has to use a present perfect - there is no flexibility there. A few extra points about the idea of interchangeability: *Most AmE speakers know that the more 'proper' form is "Have you eaten yet?", so they aren't truly interchangeable in that sense. It's more that substituting the past simple for the present perfect is widely done and quite acceptable. Among AmE speakers, nobody would notice whether you said "Have you eaten yet?" or "Did you eat yet?", whereas "Did you eat yet?" might sound odd to most BrE speakers. And if you'll forgive me for stating the obvious: please don't think that 'interchangeable' means that the present perfect and past simple are 'the same' in some way, or that AmE speakers use them randomly, substituting one for the other whenever they feel like it. You can't randomly drop in a present perfect where a past simple is required: for example, "I've written that essay yesterday" is wrong wherever you are. One last point: With increasing globalisation, the situation is changing. Young British people ( and Irish, Australians etc) are tending more and more towards the American model. It now isn't at all unusual for a 20-something Briton to say "You ate already, right?". Language changes all the time. That's what's so fascinating about it.