I've got the difference between that tenses on a basic level (remembering the appropriate time markers helps me). But in some cases, I can't get the picture which the tense is more appropriate. Let's have a look at some example.
You have already received our letter, haven't you?
Yes, we received it last week and our specialists have studied it very carefully
Why should I employ the present perfect in the second part of the sentence (it is emphasized by italic)? Isn't it more reasonable to use the past simple there as in the first part? (mentioing by that that the specialists stydied it last week too).