99495
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](https://japanese.stackexchange.com/questions/99483/the-
grammar-
of-%E6%95%99%E6%8E%88%E3%81%8C%E3%83%8F%E3%83%AA%E3%83%BC%E3%82%92%E3%81%B2%E3%81%A8%E3%82%8A%E5%8D%A0%E3%82%81%E3%81%AB%E3%81%AF%E3%81%A7%E3%81%8D%E3%81%AA%E3%81%8B%E3%81%A3%E3%81%9F)
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.
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grammar potential-form
When is it mandatory to use を with the potential form?
It depends on the verb. In this case, the noun portion of the する-verb 独り占め
contains another verb, 占める. I think this is making us feel ハリーが or ハリーを is
more strongly associated with the action of that verb than with できる.

教授はハリーが独占できなかった sounds slightly less bad because we don’t feel as much verb
(at least not in form) from 独占 as from 独り占め.

* * *

And ハリー being a person obviously increases the probability of it being a
subject when marked with が.
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answer
_" 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."_

This is correct because this sentence is not using potential form. Although
dekiru is commonly used to explain when things "can" happen, it is still using
regular old present tense, so the direct object must be marked with を as with
any sentence in present form.

The source that told you が marks objects in potential form is wrong. が
_always_ marks the subject. Look at the two sentences below:

Food can be eaten by me. Subject (が) = "Food"; No direct object. Agent = "me".

I can eat food. Subject (が) = "I"; Direct object (を) = "food"

These are different, but English translators of Japanese will use both
interchangeably, leading to the myth that the rules for が and を are arbitrary
and mysterious, and that for this verb tense the particles can change their
roles, when in reality the existence of one or the other particle indicates
which of the two above sentences the Japanese person meant, if they're there
at all.
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answer