We use the past progressive in order to talk about the duration of an event in the past, but not in its duration. 
- Steve was having breakfast at 7:30 am (We know that something was going on but we have no idea if the action was finished or not).
If we want t express the idea that an event was completed in the past, we preferably use simple past.
- Steve had breakfast this morning at home. (we know that something started and finished in the past)
Be careful, non action verbs like: like, seem, want, own, believe, prefer, possess, cannot be used with progressive forms.
Sometimes we will have statements in which an action will be interrupted by an event in the past. So, the combination of simple past and past progressive is possible.
- Steve was having breakfas when the phone rang. Steve was in the middle of an action when and event occured.
- Whe use "while" when we introduce the action.
- We use "when" when we introduce the event in the past.
Sometimes two actions can take place at the same period of time. "While" will be use to introduce any of the caluses.
- Steve was having breakfast while Joan was listening to the news.
- While Steve was having breakfast, Joan was listening to the new.
