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?

Question

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?

Answer 1

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
  • Particle は replacing を - where does the stress lie?
  • What is the subject of this sentence? Is it the book (mentioned) or the author (who is not mentioned)?
• 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.
• Exception (3): Using two は in one sentence is fine in a negative sentence. 私は日本語は話します is still grammatical, but the second は will have a strong contrastive meaning.

Answer 2

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)?