1) action was in progress at special time in the past

Peter was reading a book yesterday evening.

She was listening to the radio.

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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.

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3) together with the Simple Past

While we were sitting at the breakfast table, the telephone rang.

Note:

Past Progressive: were sitting at the table

Simple Past: the telephone rang.

The action in the Simple Past interrupted the action in the Past Progressive.

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4) repeated actions irritating the speaker (with always, constantly, forever)

Andrew was always coming in late. (I don't like it.)

Simple Past:

Andrew always came late. (Here I don't give a comment.)