I've realized that I don't really use the present perfect and simple past correctly. My main problem is that when doing exercises I've found the explanations to be (sometimes) forced.
Some of the rules for using present perfect are:
- Connection to the present
- Recently completed
- Imprecise time reference
Yet, an example like
The police arrested two people early this morning.
uses the past simple. If it was from a news channel I would find it to be correct, but if I were talking, why not use the present perfect?
I could be scared by this, so it has a connection to the present. Plus this morning could be interpreted as recently completed which is imprecise.
The reason given for the answer to this exercise is that it's a finished action (which I also find silly, because finished actions can also be expressed with the present perfect).