To me the difference here is emphasis. Are you emphasizing the past action, or the present result. " -- simple past, emphasis on the past action of buying.
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AnonymousOk, but can I say: "I've dropped my calculator but it seems ok"? The situation takes place in a class for example, and I report about it after droppingI would say, "I dropped..." in this circumstance using the simple past form. As you indicate in your post you're merely reporting about it after the fact, not concerned with its present situation.
AnonymousThe problem is I don't see the clear difference.I don't either. These book examples are always short on explanation. And besides that, they expect the non-native to have the same intuitions as a native speaker about the explanatory clauses they add to the original text.