Form
Subject + was/were + Verb-ing
The man was walking the dog when I saw him.
We were eating dinner when the phone rang.
Meaning
Action Interrupted by Another Action
Used for a longer action that was in progress in the past when a shorter action happened and interrupted it. The longer action is Past Continuous, the shorter is Past Simple.
PastPresentFuture
x
I was reading a book when she called.
While they were having dinner, the power went out.