Learn Spanish (Latin America): Todavía no he repasado para el examen.

Todavía no specifically means "not yet" or "still haven't." In Spanish, it often goes right before the conjugated verb (he). You can also split it and put the todavía at the end of the sentence (No he repasado para el examen todavía), but putting it at the beginning is very common and sounds very natural.

Why are we using he repasado instead of the simple past tense repasé?

He repasado is the present perfect tense, meaning "I have reviewed." We use it with triggers like todavía no because you are talking about an action that hasn't happened yet, but the possibility is still open right now (and you probably intend to do it soon).

Why is it para el examen and not por el examen?

We use para here because it shows a purpose, goal, or deadline. You are reviewing with the specific goal of being ready for the exam. A good trick is to think of para as meaning "intended for."

The word repasado looks like it could mean "passed". What does the verb repasar actually mean?