Learn Spanish (Latin America): ¿Te enteraste de lo que pasó?

The verb enterarse (to find out / to hear about) always requires the preposition de when you mention what you found out. This is a governed preposition, meaning the verb naturally demands it. You can't say Te enteraste lo que pasó; it must be Te enteraste de lo que pasó.

Why is there a te at the beginning? Can I just ask ¿Enteraste de lo que pasó?

No, you must include the te. The verb used here is enterarse, which is a pronominal verb (meaning it requires a reflexive pronoun like me, te, or se). While the base verb enterar exists (meaning "to inform someone else"), using the pronoun changes the meaning to "to find out" or "to inform oneself."

Why is it lo que instead of just qué for the word "what"?

In Spanish, when "what" is not asking a direct question (like "What is that?"), but instead means "the thing that," you use the phrase lo que. In this sentence, you are literally asking if they found out about the thing that happened. Notice also that it does not have an accent mark, unlike the question word qué.

Why is pasó in the preterite tense here instead of the imperfect (pasaba)?