The present perfect is a present tense so, it only tells us about the situation now. It is an update of something that started in the past.
- I have known Julie for ten years.
(I started to know Julie in the past, but we use the present perfect because we are talking about the present situation – now is ten years after I first met Julie.)
- My brother has been to Mexico three times.
(We use the present perfect because at this moment he has visited Mexico three times.)
- I have visited Tanya once this week.
(The week is still happening, so it is at this moment.)
The past simple is a past tense so it only tells us about the past.
- I spoke French when I was at school.
(I am 35 years old now, and I am not at school. Finished time, finished action.)
- My grandfather went to Mexico three times.
(My grandfather is dead, and it is not possible for him to go to Mexico again.)
- I saw Conrad last week.
(It talks about last week, and last week is finished.)
- I fed my cat some special food, so he has grown very big.
(fed is in the past simple because the action of feeding has finished. has grown is in the present perfect because he started to grow in the past and he is still growing now.)
- Yesterday we enjoyed a lovely walk. Today it has been very windy, and the walk hasn’t been nice.
(Yesterday is finished, Today is at this moment.)