TENSES

PRESENT PERFECT OR SIMPLE PAST?

Always use the present perfect when the time is not important, or not specified.

Always use the simple past when details about the time or place are specified or asked for.

Compare:

Present perfect	Simple past
I have lived in Lyon.	I lived in Lyon in 1989.
They have eaten Thai food.	They ate Thai food last night.
Have you seen 'Othello'?.	Where did you see 'Othello'?
We have been to Ireland.	When did you go to Ireland?

There is also a difference of attitude that is often more important than the time factor.

"What did you do at school today?" is a question about activities, and considers the school day as finished.

"What have you done at school today?" is a question about results - "show me", and regards the time of speaking as a continuation of the school day.