The present perfect is a 'linking tense' (like a bridge) connecting to the present, whereas the past perfect is a 'linking tense' connecting to the past. Let's say you live in London now. You moved there in 2012. So you would say: 'I live in London. I've lived here since 2012'. You use the present perfect here to talk about the time period which began in 2012, and has continued into the present. This tense links the past (2012) with the present time. The past simple describes a situation previous to a time in the past. It's the same form and use as in Italian. For example: When I came to London in 2012, it was my first time outside Italy. I had never lived abroad before. This form connects a past time (2012) with a time before that point (the years before 2012). I hope that makes sense. Please ask if anything's not clear.

If you're absolutely sure that your confusion is between Present Perfect (have seen) and Past Perfect (had seen), then Past Perfect is only used in relation to other past tenses such as Simple Past and Past Continuous. Present perfect is used only for the present (specifically: using a past event to describe or connect to the present). We never use Present Perfect to describe the past.

Do you actually mean that you can't understand the difference between the Present Perfect and Past Perfect? Or is your difficulty deciding when to use the Present Perfect and Past SIMPLE? ie the difference between 'I saw' and 'I have seen?' Let us know what the problem is, and we'll try and help you.