Respuesta:
1. Anna/walk to school/she/ see / accident.
While Anna was walking to school, she saw an accident.
2. we/play/soccer/start/snowing.
We were playing soccer when it started snowing.
3. elevator door / close / Peter/run in.
The elevator door was closing as Peter ran in.
4. yesterday / alarm /go off / I / get out of bed.
Yesterday, the alarm went off and I got out of bed as soon as it did.