Quote Originally Posted by tiudavidharris View Post
why perfective?

and what does 'был' mean? how does it function?
Perfective aspect, in addition to obtaining results, often addresses subsequent actions. If you use imperfective, then you are stating that the actions were occurring at the same time.
If you used the imperfective it would mean, as Bob said, "If she were calling (me) on Wednesday, I would be walking to school."
The use of perfective would mean more along the lines of, "If she would have called me on Wednesday, I would have gone to school."

The word бы is translating as 'would' in this example. It is an example of conditional verb usage.