Past Simple versus Past Continuous


The past simple and the past continuous do not mean exactly the same. Compare:

When Mary arrived, we were having dinner.
This means that we had already started dinner before Mary arrived.


When Mary arrived, we had dinner.
This means that Mary arrived and then we had dinner.

-What were you doing when you heard the crash?
-I was reading the paper.

-What did you do when you heard the crash?
-I ran to the window.

More examples:
I was dreaming when you woke me.
The police stopped him when he was driving home.