Use: 1. events that were taking place for a periode of time in the past. It's also finished. 2. an event in the past that got interrupted by another event. 3. two events that were taking place at the same time (in the past).
Form: to be (was/were) + verb+ing
1. I was studying for a test earlier.
2. I was cooking dinner yesterday, when the delivery guy rang the doorbell.
3. I was eating while my mom was doing the cleaning up.