You likely know the present continuous ("I am sitting at the table," "He is going to the grocery store"), but how familiar are you with the past continuous?
The past continuous is often used to set the scene and provide context when talking about the past:
I was working in the theater in England.
Caption 13, Donald Sutherland: talks career and Hollywood
A very typical structure with the past continuous and simple past tenses together occurs when one action or event in the past interrupts another action that is already in progress.
There she encountered an old woman who was sitting at a spinning wheel.
Caption 32, Fairy Tales: Sleeping Beauty
In this example, it is clear that the woman already began doing what she was doing ("sitting at a spinning wheel") before Sleeping Beauty entered the room. In each of the following examples, one action was already happening when the other occurred:
But when the Princess opened her eyes the next morning, she was surprised that a good-looking prince was standing there.
Caption 26-27, Fairy Tales: The Frog King
I'm sorry, I was eating chips. What did you say?
Caption 12, The Ellen Show: Ellen Inspired Adele’s New Song
To get more context for the phrases, watch the videos above on Yabla English. Make sure you understand which action came first. For a thorough description with more examples, you can also refer to this page.