To describe an action in progress at a specific moment in the past .
To describe the situation at the beginning of a story or narrative.
When she was a
baby...
For a complete action in the past. I was working in my office.
Use the past continuous to indicate that a longer action in the past was interrupted. The interruption is usually a shorter action.
I was watching TV when she called
The past continuous with words such as "always" or "constantly" expresses the idea that something irritating or shocking often happened in the past. She was always coming to class late.