Differences Between the Preterite and the Present Perfect

In English we can use either the Simple Past or the Present Perfect to express a completed action in the past:

Yesterday I studied hard.
I have studied hard.

In Portuguese, only the Preterite expresses a fully completed action in the past. The Present Perfect expresses an action which began in the past , has been developing over a period of time or may or may not continue into the future.

Eu encontrei a Maria na Biblioteca.
I met Maria in the Library / I have met Maria in the Library.

Eu tenho encontrado Maria na Biblioteca.
I have been meeting Maria in the Library. (and I may still continue meeting her in the Library).