(Please note that British and American English have different rules for the use of this tense. The explanation and exercises here refer to British English. In American English, it is often acceptable to use the past simple in some of these examples.)
We use the present perfect when we want to look back from the present to the past.
We can use it to look back on the recent past.
When we look back on the recent past, we often use the words 'just' 'already' or the word 'yet' (in negatives and questions only).
It can also be used to look back on the more distant past.
When we look back on the more distant past, we often use the words 'ever' (in questions) and 'never'.