We often use the Past Continuous when we refer to an action that was happening in the past when another one interrupted it.
ex: He was listening to music when someone knocked at the door.
Affirmative: He was singing a nice song.
to be (past simple) + verb+ing
Negative: He wasn’t singing a nice song.
to be+not (past simple) + verb+ing
Interrogative: Was he singing a nice song?
to be (past simple) + subject) + verb+ing