Quote Originally Posted by tiudavidharris View Post
why perfective?
Really, it is hard to explain why. I just know how I would say it. Your version with imperfective (Если бы она звонила в среду, я бы шёл в школу) is strange and does not sound natural.

I would try to provide some explanation, but I am not sure if this explanation is perfect.

The past tense imperfective emphasizes the action in its process (i.e. how the action was performed), i.e. it is a point of view "from inside the action".
Если бы она звонила в среду, я бы шёл в школу - it is more like "If she were calling (me) on Wednesday, I would be walking to school".

BTW, is your native language English? I wonder if my explanation makes sense to you

But in your example, you actually need the action "as a whole", i.e. "looking on it from outside". That is why you need perfective.


Quote Originally Posted by tiudavidharris View Post
and what does 'был' mean? how does it function?
There is no "был" in this sentence. There is "бы". They are completely different words!

"бы" is a particle (it has no exact equivalent in English), but it is used to form conditionals. So, a verb with "бы" refers to a possibility, not to a real action. Something which could happen but did not.

A simple example. Compare:
Я тебе помог - I helped you (it really happened)
Я бы тебе помог - I would help you (but I did not)
Если бы ты меня попросил, я бы тебе помог - If you have asked me (but you have not!), I would help you (but I did not!).