Learn Spanish (Spain): Mientras paseaba por el parque, me encontré con mi hermano. - While I was strolling through the park, I ran into my brother.

This is a classic example of setting the scene versus an interrupting action. paseaba (was strolling) is the ongoing background action, so it uses the imperfect. me encontré (ran into) is a sudden, completed event that happened during the stroll, so it uses the indefinido.

Why does the sentence use me encontré con instead of just encontré?

The verb encontrar means to find something. But when you add the reflexive pronoun me and the word con to make encontrarse con, the meaning changes to bumping into or running into someone by chance.

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

When talking about physical movement, por means through, around, or along an area. para would mean the park is your final destination, but here you are just strolling through it.

Since paseaba could mean I, he, she, or it was strolling, how do we know it means I?

Because the second half of the sentence uses me encontré (I ran into). Since the same person is doing both actions, context tells us that the hidden subject for paseaba is also yo (I).