THE PAST CONTINUOUS TENSE

The past continuous tense (also called the past progressive tense) is commonly used in English for actions which were going on (had not finished) at a particular time in the past. The past continuous tense expresses action at a particular moment in the past. The action started before that moment but has not finished at that moment.

We often use the past continuous tense to "set the scene" in stories. We use it to describe the background situation at the moment when the action begins. Often, the story starts with the past continuous tense and then moves into the simple past tense.

How do we make the Past Continuous Tense?
The structure of the past continuous tense is:

subject + auxiliary verb BE + main verb
was base + ing
were


Examples :

Michael was washing his car when his father phoned him.
Suzie was reading a book when Melissa came to her house.


How do we use the Past Continuous Tense?

The past continuous tense expresses action at a particular moment in the past. The action started before that moment but has not finished at that moment.When we use the past continuous tense, our listener usually knows or understands what time we are talking about.

Look at these examples:
I was working at 10pm last night.
They were not playing football at 9am this morning.
What were you doing when he arrived?
We were having dinner when it started to rain.


Source :http://www.fjwu.edu.pk/blog/past2.htm