What's the difference between saying 私は日本語を話しません vs 私は日本語が話しません?


> 私は日本語を話しません
>
> 私は日本語が話しません

I've seen both versions on the Internet. Maybe only one is the correct
version? Maybe both?




日本語が話しません is ungrammatical. It sounds very unnatural. The following is the
summary of what is OK and what is not:

>   * 私は日本語 **を** 話しません: OK
>   * 私は日本語 **は** 話しません: OK
>   * 私は日本語 **が** 話しません: **Wrong!**
>   * 私は日本語 **を** 話せません: OK
>   * 私は日本語 **は** 話せません: OK
>   * 私は日本語 **が** 話せません: OK
>

話す is a transitive verb (like English "speak"), so basically its object must
be marked with を. But there are several exceptions you have to consider.

  * Exception (1): When the object is decorated by the topic marker, をは turns into は (or は replaces を). 
    * [は is used for Objective 格助詞](https://japanese.stackexchange.com/q/52336/5010)
    * [Particle は replacing を - where does the stress lie?](https://japanese.stackexchange.com/q/14708/5010)
    * [What is the subject of this sentence? Is it the book (mentioned) or the author (who is not mentioned)?](https://japanese.stackexchange.com/q/17571/5010)
  * Exception (2): When the verb is potential, the object can be marked with が, too. (話せる is the potential form of 話す) 
    * [The difference between が and を with the potential form of a verb.](https://japanese.stackexchange.com/q/609/5010)
  * Exception (3): Using two は in one sentence is [fine in a negative sentence](https://japanese.stackexchange.com/q/1077/5010). 私は日本語は話します is still grammatical, but the second は will have a strong contrastive meaning.




私は日本語を話しません means "I don't speak Japanese". 日本語が話しません is unnatural because
this が is the subject marker, so it means "Japanese language doesn't speak".
However 日本語は話しません make sense.

This usage of は is the contrastive-marker function. The difference of が and は
is already explained in this. [What's the difference between wa (は) and ga
(が)?](https://japanese.stackexchange.com/questions/22/whats-the-difference-
between-wa-%e3%81%af-and-ga-%e3%81%8c)

