"Saat yedide okula gitti."

Translation: She went to school at seven o'clock.

[JosefKnecht.]

how do I know that she does the action?

[Selcen_Ozturk]

gitti: she went

the verb is conjugated according to the subject

[vegan_zombie]

You don't. The subject in the sentence is "o," who/which could be a he/she/it. DL would accept all three.

[Dragonrykr]

It sounds like sağ