We use the past continuous to talk about something was in progress at a past time. The action or situation had started but it had not finished at that time.
I saw you last night. You were waiting for a bus.
Was Sue working at 10o'clock yesterday morning?
Compare the uses of the past continuous and past simple:
Past Continuous:
I was writing a letter.(= I was in the middle of writing)
Past Simple:
I wrote a letter.(= I started and finished it.)
The past continuous describes a longer action or situationwhich was in progress at a past time; the past simple describes a shorhet actionswhich happened in the middle of the longer action, or interrupted
