Learn Spanish (Latin America): Ellos estaban buscando el refugio cuando el rayo cayó. - They were looking for the shelter when the lightning struck.

We use the past continuous (estar in the imperfect tense + the -ando ending) to talk about an action that was actively in progress in the past. It sets the scene: they were right in the middle of looking for the shelter when something else happened.

Why isn't there a word for 'for' after buscando?

In Spanish, the verb buscar automatically means 'to look for' or 'to search for'. You never need to add a preposition like por or para after it. So, buscando el refugio completely translates to 'looking for the shelter'.

Why is cayó in the preterite tense while estaban buscando uses the imperfect?

This is a very common storytelling pattern! The past continuous (estaban buscando) gives the ongoing background action. Then, the preterite tense is used for the sudden, completed action (cayó) that interrupts it.

Wait, doesn't cayó come from caer, which means 'to fall'? Why does the translation say 'struck'?

Yes, exactly! In English, we usually say that lightning 'strikes', but in Spanish, the natural way to describe this is that it 'falls'. So el rayo cayó literally means 'the lightning fell', which is how Spanish speakers say 'the lightning struck'.