Ответ:

1. What were you doing when the storm started?

I was watching TV while Ann was reading a novel.

2. My sister was looking out of the window this morning when she saw an accident.

Really? Did anyone get hurt?

3. How did they get lost in the forest?

They lost their map, and they started walking in the wrong direction.

4. As the train was leaving the station, it caught fire.

That's awful. Are the passengers OK?