Question

w 1. The students were doing an exercise the bell rang. 2. I saw an accident I was waiting at the bus stop. 3. she was writing an email, her computer turned off. 4. He fell down he was climbing a tree. 5. my mom came home, my sister was sleeping. 6. We found some old photographs we were tidying our room. 7. you called me last night, I was studying for the test. 8. I had a bad dream I was sleeping on the sofa. 9. the fire alarm went off, we were listening to music. 10. She started to cry she was watching a movie.
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Answer 1

1. when
2. while
3. when
4. while
5. when
6. while
7. when
8. while
9. when
10. while

while se usa para algo qué pasa mientras otra acción sucedía simultáneamente.
when se usa para una acción que sucede después de otra, como en cadena.