Mi abuelo empezó a roncar en el sofá mientras veíamos la película.

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Breakdown of Mi abuelo empezó a roncar en el sofá mientras veíamos la película.

• nosotros: we
• a: to
• en: on

Questions & Answers about Mi abuelo empezó a roncar en el sofá mientras veíamos la película.

Why is it empezó and not empezaba?

Because empezó is the preterite, and here it describes a completed event that happened at a specific moment in the past: your grandfather started snoring.

In contrast, veíamos is imperfect because it describes the background action already in progress: we were watching the film.

This is a very common contrast in Spanish:

• preterite = the event that happens / begins / interrupts
• imperfect = the ongoing background situation

So the sentence sets up this idea:

• while we were watching the film → ongoing background
• my grandfather started snoring → one event that happened during it

Why is it empezó a roncar? Why is there an a after empezó?

Because empezar is commonly followed by a + infinitive when it means to begin/start doing something.

So:

• empezó a roncar = he started to snore
• empezó a hablar = he started to speak
• empezó a llover = it started to rain

This is just the normal pattern in Spanish:

• empezar a + infinitive

You cannot usually leave out the a here.