What are the differences between "told" and "have told"?
How do you know that?
Jack told me so.
Can we use "Jack has told me so" instead, since it is related to the present?
Why are you fighting?
Because he hit me.
Why cannot I use "because he has hit me", since it is related to the present?
Many past events have present relevance without the necessity to use the present perfect tense. For example:
I ate too much for breakfast and now I feel sick.
In this case the phrase for breakfast sets the eating at a clearly identified moment in the past and therefore the past tense is used.
A more explicit past tense reference is seen in the following sentence:
I came to London a year ago and I never want to leave.
Clearly, the present perfect does not work with such explicit past / finished time references.
However, not all statements about the past contain explicit references. What is important is whether or not the speaker conceives of the event as being a completed past event. So, if I'm focused on the moment in the past when I got the information from John, then I would say John told me ... .
If, on the other hand, I am more concerned with conveying the fact that my present knowledge results from John telling me, and I am not focused so much on the moment when he did so, then I say: John's told me ... .
The same applies to hitting. This is usually a very short dramatic action and it is not surprising that the implicit focus will be on the completed moment in the past. That is why the past tense seems more natural. But it is possible to imagine a context where the present perfect could be used. For example, you turn the corner and witness a fight. You ask a bystander what's going on and she replies: Well, John's hit Fred and now they're fighting.
This is a very tricky aspect of English grammar for non-native learners. But in general, the intuitive, spontaneous choice made by native speakers in such contexts will be past simple if the action is clearly set (explicitly or implicitly) in the past. Conversely, the present perfect will be chosen if the moment when something occurred is irrelevant, only its present result.
