Sometimes they say that if we have a result of a completed action in the present, we use Present Perfect. However, I need to clarify this moment. First of all, it’s not an absolute rule. Today we can use Past Simple almost all over. We can’t use Present Perfect at all times. Definitely you need to see Present Perfect and Past Simple Compared: When we don’t use Present Perfect
We prefer to use Present Perfect if there is a result of a completed action.
So, how do we know if there is a result of a completed action. Compare:
The situation has changed. (Now the situation is still different)
The situation changed. (But now it is back to normal)
She has lost her keys. (She doesn’t have her keys now)
I lost my keys. (But now I have my keys or even duplicate keys)
I have lost my keys. (Even now I have duplicate keys, but those keys were so special and I don’t have those keys now)
What did you do today? (Asking about activities)
What have you done today? (Asking about results)
That is how we distinguish if there is a result of a completed action. And we are done!