Задайте вопросы в past continuous с исходного предложения:
1. They were walking home when the rain began.
Они шли или бежали домой, когда начался дождь?
2.When father came from work, mother was putting Mary in bed.
Что делала мама, когда папа пришёл с работы?
Откуда пришёл папа?

Were they walking or running home when the rain began. Where did father come from. What was mother doing when father came from work