The present perfectThe aspect expressing completion. E.g.: He’s finished university. tenseA grammatical category that is marked by verb inflection. To express when an event or action happens in time or when a state exists, we use tenses. is used for recent actionA general term expressing that somebody or something does somethings that have a result in or an effect on the present.

Someone has opened the window.

Here we want to emphasisSpecial importance or attention given to something. E.g.: All I wanna do is have some fun.e that the window is open. The sentence suggests that it is probably cold inside. Compare with this one:

Someone opened the window.

This sentence simply states a fact. Whether the window is open or not is not important. The focusThe main or central point of attention or interest; what is emphasized. E.g.: All I wanna do is have some fun. ("all") is on the action, not the result or the effect of that action.

Diagrams

Present perfect for past actions
Present perfect Present perfect with an incomplete period

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