A- Present Perfect.
Simply, the present perfect tense has many aspects of use.
1- when the time is not important and you want just concentrate on the action.
e.g. I have eaten my dinner.
In the sentence you want to tell that you have eaten already. That's it.
2- When you do more on thing more than one.
e.g. I have taken three exams so far.
In this sentence you want to tell the times you have taken the exams.
3- When you bave tried something before or not.
e.g. I have been in Canda.
In this sentence, we understand that you have beeb in Canada before.
or you can say ' I haven't been in Canada.
B. Past Simple.
You concentrate on time.
e. g. I ate my dinner last night.
I know my English is poor but I was trying to help in easy way. I hope that helps.