Learn Spanish (Latin America): El invasor intergaláctico destruyó la ciudad mientras tú roncabas. - The intergalactic invader destroyed the city while you were snoring.

This is a classic example of a completed event happening during an ongoing background action. The snoring (roncabas) was an ongoing process in the past, setting the scene. The destruction of the city (destruyó) is viewed as a single, completed event that happened while that background action was taking place.

Why is the pronoun tú included here instead of just saying mientras roncabas?

In Spanish, subject pronouns are usually dropped, but adding tú here creates emphasis and contrast. It highlights the dramatic (and funny) difference between the two subjects: the invader was doing something huge, whereas you were just sleeping through it.

Could I use the past progressive here, like mientras tú estabas roncando?

Yes, you absolutely can! Saying mientras tú estabas roncando means exactly the same thing. However, native speakers frequently use the simple imperfect (roncabas) to express an ongoing past action, especially after the word mientras (while).

Why does intergaláctico have a written accent mark?

Intergaláctico is an esdrújula word, meaning the spoken stress falls on the third-to-last syllable (-lác-). In Spanish, all words with this stress pattern require a written accent mark because they break the natural rule (which is to stress the second-to-last syllable for words ending in a vowel, 'n', or 's').