What is the difference in: I learned that song and I've learned that song? I'm really confused by past tense and past participle.
Past Simple tense (I learned) tells only about the fact that occured in the past, while Present Perfect tense (I have learned) indicates a connection between the fact in the past and the present situation.
If I say I lost my keys. (in Past Simple) I do not really tell if I have the keys now. The sentence says only that in some moment in the past I lost them, but later I could have found them.
The sentence I have lost my keys (in Present Perfect) indicates not only the fact of losing the keys in the past, but also the relation between this past event and present situation. This sentence underlines the fact that now I do not have my keys.
Note. Past participle is not the name of tense. It is the form of a verb used to create the sentences in some tenses: Perfect tenses, Passive Voice, etc.