Past tense versus Present perfect

SENTENCES:

1. What are you doing? I (tell) ______ you not to touch my suitcase!

2. What an unusual print! Where (you find) __________ it?

In both cases the grammar book gives me as keys the simple past ("I told you..." and " did you find it?") and so I told my students.

Anyway some of them put the present perfect and asked me why their answers were uncorrect. They said they chose the ppbecause there's a link between present and past. I was unable to give them any explanation...

Actually, I'm in two minds too, especially regarding sentence n.2, but my students need an answer to clarify their doubts (and so do I...) :-(

What I ask you is:

- as for sentence n. 1, I would use the simple past just because it sounds me better, but is the pp uncorrect? If so, why?

- as for sentence n. 2: me, too, I would have used the present perfect, because the first part of the sentence shows me the print is there, before me.... Why is the past tense the only correct tense, instead? What' s the rule behind?

Thank you in advance for your help!

ps. Maybe I posted it twice. Sorry.

PlP