Rule

Ask this way: Was he doing?

No, it didn't happen: No, it was not happening.

When to use?

1. When we know the time: At seven o’clock I was cleaning the house, so I could not kill the king.  

2. Compare, and use past continuous for the longer happening.

I was sleeping when he climbed into my room.

3. When doing something took a long time: I was cleaning the house all day.