Past Progressive Tense

The PAST PROGRESSIVE TENSE indicates continuing action, something that was happening, going on, at some point in the past. This tense is formed with the helping "to be" verb, in the past tense, plus the present participle of the verb (with an -ing ending):

• I was riding my bike all day yesterday.
• Joel was being a terrible role model for his younger brother.

The past progressive indicates a limited duration of time and is thus a convenient way to indicate that something took place (in the simple past) while something else was happening:

• Carlos lost his watch while he was running.

The past progressive can express incomplete action.

• I was sleeping on the couch when Bertie smashed through the door.

(as opposed to the simple past, which suggests a completed action:

• I slept on the couch last night.

The past progressive is also used to poke fun at or criticize an action that is sporadic but habitual in nature:

• Tashonda was always handing in late papers.
• My father was always lecturing my brother.

Generally, progressive forms occur only with what are called dynamic verbs and not with stative verbs.

Reference : http://grammar.ccc.commnet.edu/grammar/tenses/past_progressive.htm