Where were you going when I saw you this morning?
Hannah was reading a book when Gary called at her house.
I was not working when Dave came to visit us.
Were you waiting at the bus stop when it started to rain?
The teacher was shouting at us because we did not concentrate in class today.
Were you having dinner when I phoned you?
Если предложение начинается с when...(Simple tense), +подлежащее (Continuous tense).ставится запятая.
When I saw her, she was doing her homework.
НО!
She was doing her homework _when I saw her.(запятая не нужна)
