I don't know what I should use in this situation.
I will give you an example:
Mary drew / was drawing a family picture while her grandparents were staying in Japan.
What is correct?
The choice of the Tense depends on the character of the action of 'drawing'. If she was in the process of drawing the picture, you should use the Past Continuous. If she finished the picture during that period, you should use the Past Simple.