present perfect3

[bayan said]

Dear sir,
Thank you very much for using your brilliant style at writing & answering.This time there're some sentences about which I've questions:
1-why is it possible to use both the past simple & present perfect in this sentence"I`m afraid I tore/have torn the news paper so it`s difficult to read"
2-"Have you ever seen a magic show?~no,but my sister saw David copperfield"why is the past simple used with"saw"& not the present perfect although the time isn`t mentioned?
with my best compliments..
Bayan..

[Tdol]

1 I think that I would use the past if the tearing had occurred a couple of hours earlier, and the present perfect for a recent tear.
2 You could use the present perfect, but the past suggests that the speaker is thinking of it as as an event in the past, maybe with a mental idea of it being two years earlier, or whenever.