- Action interrupted by another action
=> long action: use while + Past Continuous
=> short action: use when + Past Simple
They were watching a movie when the phone rang.
While he was driving, the car broke down.
When the car hit him, he was walking.
- Actions in progress at the same time
While Lidia was cleaning the house, Dan was cooking dinner.
Maria was reading while George was sleeping.
*** NOTICE THE DIFFERENCE IN MEANING:
1) When he came home, she cooked dinner. (he came home, and she started cooking dinner after that)
2) When he cam home, she was cooking dinner. (she started cooking dinner before he came home)