 The main function for the present perfect tense is to relate something in the past to the present. We can do so in a number of ways. First, we have indefinite past actions. I have been to Italy twice. We're not concerned with when it happened. We just simply want to say that it has happened in the past. It's a fact of something I've done in the past, but yet it's still true in the present. Unfinished past actions. I have lived here for three years. I started living here in the past, and it's still true now. With this usage, you will typically see time expressions. Finally, we have past actions with present results. I have lost my keys. It's implied that I still haven't found them. I lost them in the past. I don't have them now. I've lost my keys. There are further usages. They're not as common as these three. I suggest you look in your materials for those further usages.