We use the past progressive and the past simple in the same sentence when one action interruptedanother in the past. We use the Past Progressive for the longer action and the Past Simple for the shorter action. In this case we usually use while, when or as. As/While I was driving, I saw a cat in the street.
I was sleeping when the telephone rang.
While the snake was hangingin the air, he cut it with a knife.
I was feeling dizzy and sick while I was drinking it.
As soon as I finished they all looked at me for a response.
The two actions were happening at the same time.
The two actions happened one after the other.
One action was in progress when the other happened.