The Tense is used to indicate that an action was continuing in the past.
Also this tense is used to show that while an action was finished in the past another action is still going on.
Sub + was/were + verb+ing + Obj
Example
- When I came into the room, she was reading a novel.
- When Tom reached home, his child was waiting for him at the gate.
- His children were sleeping when she came.
- The light went off while I was reading.
- I was playing chess while he called upon me.