UNIT 2 Past continuous

The past progressive is used for:

• actions that were in progress at a specific point of time in the past

• an action that was in progress in the past when it was interrupted by another action (the action that interrupted the one in progress is in the past simple)
• actions that were happening at the same time in the past.
• background information.

examples:
I was watching a movie at ten o'clock last night.
We were having dinner when the lights went out.
While Liz was preparing lunch, Frank was watching the news.
The sun was shining and the snow was melting slowly.
for more examples:
http://www.ego4u.com/en/cram-up/grammar/past-progressive/exercises
http://www.englishpage.com/verbpage/verbs3.htm