The primary meaning of the Past Continuous is that of a past action shown in its progress at a given past moment, e. g.:

• The door was slowly opening, and Anthony found himself gazing into a pair of pale-gray hooded eyes. (Gordon)
• She followed his gaze through the falling rain and saw a man and a girl coming from the large block of flats opposite her home. Now they were getting into a little motor car. (Gordon)

I found this comic strip especially suitable to learn/teach the differences between simple and progressive past and present perfect (I’ll use it the classroom as part of the lesson plan on past tenses):