Past simple versus past continuous

Well, the first thing to know is that these two tenses are very different, the simple past describes FINISHED actions in the past while the past continuous describes actions in PROGRESS in the past. When we use these tenses together it means that two actions happened at the same time, in other words one started first (the past continuous action), and the other interrupted the first one (the simple past action). Check the following example:

(from the homework for April 6th)

I ____ (meet) your sister at the movies. She _____ (wear) jeans and a t-shirt.

two actions are interacting or happening at the same time meet "your sister" and "your sister" wearing jeans and a t-shirt. From these two actions, one started first, first she was wearing jeans and a t-shirt; after that I meet her at the movies. So the sentence has to be completed like this:

I met your sister at the movies. She was wearing jeans and a t-shirt.