You're correct that nag is for past tense. The four tenses for this conjugation are as follows: mag +root = infinitive form (used after pseudo verbs like gusto, ayaw, pwede, dapat, etc, and for commands) nag + root = past (nag-aral, have studied) nag + first syllable of root + root = present (nag-aaral, am/is studying) mag + first syllable of the root + root = future (mag-aaral, will study) Hope that helps!

Pag-aaral is a word which means studies..You cant remove pag and say aaral in this situation....but the pag that youre refering to comes from the word "kapag" which means 'if' or whenever,depending on how you use it...Example: Kapag nagkita kami. Natives usually drop the "ka" and say 'Pag nagkita kami (If we meet). you can also use Pag when explaining something. Example: Lumalaki ang butas ng ilong ko 'pag nagagalit ako. (My nostrils grows bigger whenever I get mad.) and you can also say Kapag yung kulay dilaw ang pinili mo, pagtatawanan kita. ( If you choose the yellow color, i will laugh at you.) I Hope you can understand the context. I Cant explain it clearly.Sorry my bad, i hate Filipino subject when i was a student..lol