Please consider the following sentences:
I worked in China last year.= simple past
I was working in China last year.=past continuous.
As I understand it, the simple past is used to describe an action that started and ended at specific times. Since it started at point A and ended at point Z then isn't the intervening time between A and Z continuous? In which case, how would a learner know which of the two sentences to use since they both appear to serve the same function.
I've noticed that some books, including Michael Swann's Practical English usage book, suggest that simple past is used to indicate a short duration whereas a continuous form is used to imply a longer period of time. I personally cant see this.