action in the middle of happening at a stated past time

He was playing tennis at 4.30 yesterday.

past action in progress interrupted by another past action. The longer action is in the Past Continuous, the shorter action is in the Past Simple.

While I was getting dressed the bell rang.

two or more
simultaneous past
actions

While I was sunbathing,
Tim was swimming.
or background
description to events in
a story

She was flying to Paris.
The sun was shining...