In English, when do you use past perfect tense and present perfect tense?
Present Perfect Tense
Present Perfect is used to talk about past events which have consequences in the present.
Let's compare two sentences:
I lost my keys.
I have lost my keys.
The first sentence informs only about past event, but we do not know if the person has the keys now or not (he might have found them some time after he lost them).
The second sentence means that the keys are still lost.
Past Perfect Tense
Past Perfect tense is mainly used in compound sentences together with other past tenses (usually Past Simple). It tells the reader which of the past actions happened first, and which next.
After Tom had done his homework, he went out to his friend.
In this sentence the action expressed in Past Perfect tense (doing homework) took place before the action expressed in Past Simple tense (going out) .