Difference between を話す and で話す

Question

日本語を話す
日本語で話す

I found that those two sentences can be translated as "I speak Japanese". Is there a difference regarding the particles?

Answer 1

On the one hand, を in this case indicates the direct object. You are talking Japanese.

On the other hand で indicates the means by which you accomplish the action. You are talking in Japanese, or talking using Japanese.

Note that both sentences could be extended :

• 電話で日本語を話す : I talk Japanese on the phone (using the phone).
• 日本語で起{お}こった事｛こと｝を話す :　I talk in Japanese (about) what happened.
• 事実{じじつ}を話す : To speak the truth

Note that the second example is not the most natural, you would often use について or に関{かん}して to indicate the topic.