When we start to learn Japanese we are told that が marks the 'object' in sentences with the potential form. We expect 私は日本語を話せる but are told it should be 私は日本語が話せる. It is sometimes justified by translating as "to me Japanese is speakable" thus retaining Japanese as the subject.
Later on we find that を can actually mark the 'object' and that 私は日本語を話せる is actually fine. I must admit I still don't have a good grasp of this.
In this question I proposed the sentence 教授はハリーがひとり占めできなかった with the intended meaning of "the professor was unable to monopolise Harry" but was told that this had the opposite meaning of "Harry was unable to monopolise the professor". The sentence should actually be 教授はハリーをひとり占めできなかった.
My question is, why? Why can I not think of 教授はハリーがひとり占めできなかった as "Harry was not monopolisable to the professor" in the same way as "Japanese is not speakable for me"?
I'm further confused because "A Dictionary of Basic Japanese Grammar", when discussing を versus が for potential forms, explicitly states on page 371 that "Dekiru 'can do', however, always requires the object of an action to be marked by ga".
I think I must be missing something pretty basic here.