The present perfect connects the past action to the present in some way.
You have eaten all the ice cream. Now I can't have any.
You ate the ice cream. No wonder you're putting on weight.
I've lost my wallet. Now I have to cancel my credit cards, get a new driver's licence, and, oh, by the way, you have to pay for lunch today.
I went to Prague in the spring of 2009. It was a great trip, except that I lost my wallet.
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