Dear Vinicius,

Starting from my fair knowledge of Brazilian Portuguese, and from discussions with my Brazilian friends, I can conclude to you that since you guys do not have present perfect in your spoken Portuguese SO it is more difficult to explain the tense for you than for other people, as you won't feel when and how to use it, but you can study and apply rules on how to use it. And this doesn't happen to you guys in other tenses that you already have in your mother language ex: present perfect continuous, simple past ......

I can simply brief some cases of present perfect that I remember:  (keeping in mind that sometimes spoken English uses simple past instead just like you guys do in Portuguese and it gives the same meaning)

- When talking about something that happened since a time, or for an interval of time  ex: I have lived in Brazil for 1 year (por um ano), I've lived in Sao Paulo since 2010 (desde 2010)

- When talking about something that happened just now, ex: I've just called Jerry and apologized to him (acabei de chamar jerry....) 

- ============================= already happened, ex: I've already explained the lesson to the students and now I'm explaining for you. (ja' expliquei....)

-  Something that never happened before: He has never failed an exam, he's a genius.

-  Something that happened if you don't mentioned when it happened, ex: My uncle has studied Portuguese in Brazil. She has studied French for 2 months that's why she can understand it.

 Important Note: you must have been confused with the present perfect in English because: 

1- English present perfect = Portuguese simple past  (I've seen you before = ja' ti vi)

2- English present perfect continuous = Portuguese version of present perfect (you've been waiting for her since last year = Voce tem esperado desde o ano passado) ............... I hope I could hit the correct meaning in Portuguese as I'm still learning hahahahaha

Hope this clears out some issues for your and other Brazilian friends learning English tenses :-)