It is a complex tense (two verbs) that expreses an action completed during a period of time in the past.
The structure is the same than the present continuous, but the verb to be is in past. The rest works exactly the same.
— She was driving a lorry.
→ She wasn’t driving a lorry.
→ Was she driving a lorry?
Differences between the past simple and the past continuous:
Past simple
→ It is a simple tense (only one verb)
→ It expresses an event (punctual action) in the past.
→ In a subordinate sentence it is introduced by the nexus when.
Past continuous
→ It is a complex tense (two verbs, the auxiliary be (in past) + -ing)
→ It expresses a period of time in the past.
→ In a subordinate sentence it is introduced by the nexus while.
— I was listening to the radio when my mother called me for supper.
— My mother called me for supper while I was listening to the radio.