Past continuous

Or imperfect or past progressive.

Affirmative: He was writing
Negative: He was not writing
Interrogative: Was he writing?
Negative interrogative: Was he not writing?
This is typically used for two events in parallel:

While I was washing the dishes my wife was walking the dog.
Or for an interrupted action (the past simple being used for the interruption):

While I was washing the dishes I heard a loud noise.
Or when we are focusing on a point in the middle of a longer action:

At three o'clock yesterday I was working in the garden. (Contrast: I worked in the garden all day yesterday.)