Always use the tense to indicate a connection between actions that happened in the past.

It is either past perfect tense or pat progressive.
Example:
 past perfect--- I had cooked when they brought the food.
 past progressive---I was cooking when they brought the food.
Had cooked is past perfect, and was cooking is past progressive. Had cooked indicates cooking was finished, and wascooking indicates it was going on or in progress when something else happened (in the simple past).