1) actions were in progress at special time in the past
Peter was reading a book yesterday evening.
2) two actions were happening at the same time (the actions do not influence each other)
Anne was writing a letter while Steve was reading the New York Times.
3) together with the Simple Past
While we were sitting at the breakfast table, the telephone rang.
4) repeated actions irritating the speaker (with always, constantly, forever)
Andrew was always coming in late.
to be (was, were) + infinitive + -ing
I was playing football.
You were playing football.
I was not playing football.
I was't playing football.
You were not playing football.
You weren't playing football.
Was I playing football?
Were you playing football?