Robin2:
"I am writing you a long letter because I don't have time to write a short one"
Again: If I am asked to analyse the above quoted sentence, I would do --
The writer is making a transition from one sentence (I am writing you a long letter) to another sentence (I don't have time to write a short one). In order to make a smooth transition, the grammar helps with a transitional adverb which is because. According to syntax rules, the transitional adverb needs to be punctuated and accordingly the sentence becomes as:
I am writing you a long letter; because, I don't have time to write a short one.