why the passe simple?

Question

Why do you use the passe simple here (tu agis for 'you were acting') instead of "tu as agi" or "tu agissais" ?

Accepted Answer

Bonjour Chris,
Maarten is correct. It is not Le Passé Simple here but Le Présent Indicatif.
I believe the question you are referring to is the following: 
Depuis que tu es rentré, tu agis bizaremment = Since you came home, you've been acting strangely.
-> tu agis = You've been acting -> Présent indicatif = Present Perfect Continuous 
Conjugation - Agir (to act) 
aren-t-we-using-present-perfect-continuous-here-and-not-just-present-perfect
I hope this is helpful.
Bonne journée !

Answer

Without the full details, can only assume this is not passé simple but indicative present tense, and that the translation is not literal - which is common when ‘depuis’ is involved, because of the different tenses usually used in this context in English and French.