[chryuop]

This is how it was explained to me:

*A direct object in a sentence using potential verb form, can be followed by either "ga" or "o" or "wa"
However, each brings its own different nuance to the meaning.Thus:

A,(watashi wa) nihongo ga hanase-masu.(I can speak Japanese/ The language I can speak is Japanese.)
=> This _expression puts stress on "nihongo" which the subject(watashi) can speak.
* Question: "anata wa nani-go ga hanasemasuka ( What language can you speak?)
Answer: "watashi wa nihongo ga hanasemasu" ( I can speak Japanese language)

B,(watashi wa) nihongo o hanase-masu. (I can speak Japanese )
=> This _expression emphasizes the subject's ability to be able to speak Japanese.
* Question: "anata wa nihongo ga hanasemaska"( Can you speak Japanese?)
Answer: "hai, watashi wa nihongo o hanasemasu"( Yes, I can )

C,(watashi wa) nihongo wa hanasemasu ( I can speak Japanese.)
=> This _expression implies the subject (watashi) can't speak languages but Japanese.
(watsshi wa) nihongo wa hanasemasen ( I can't speak Japanese.)
=> This _expression implies the subject can't speak Japanese ,but other language.