We don't normally use the present perfect for definite expression of "time up to now" like
(Swan, Practical English Usage):
I saw John this week and he said...
But why in a negative and question the present perfect is normally used:
Have you seen Bill this week?
I haven't seen Bill this week.
I don't understand difference between them. Could you explain?
Expressions like "this week", "today", "this month" are a bit tricky when you need to choose between past simple and present perfect because the action is past but the time period is not over yet and without the context both are possible.
I saw John this week and he said...
Here you are talking about that particular time you met John and about what happened then. There is no reason to use the perfect tense.
I've seen John three times this week, I seem to run into him everywhere I go.
This implies "so far this week", which means that it might happen again.
Of course, you can say
I saw John three times this week.
That implies that you are quite sure you won't be seeing him again till the end of the week, for example it is already Sunday evening or you don't plan to leave the house or you have left the town where John is. Either way, those meetings are a past event.
The question and the negative are in the present perfect because they imply "so far, until now". You can also say
Did you see Bill this week? You were supposed to meet him on Thursday.
In this case we are talking about a particular occasion in the past and the past simple is more appropriate.
We normally use present perfect for a finished action with a result in the present. For example:
I've lost my keys! (The result is that I can't get into my house now).
And past simple for a finished action with no result in the present. For example,
I lost my keys yesterday. It was terrible! (Now there is no result. I got new keys yesterday).
So in your example it means that up until now have you seen Bill this week?
