In a really basic sense, が indicates the party carrying out the action of the verb, while を indicates the object/person that is directly targeted or used by the action - or, in more general terms, the argument that the verb takes.|
If you were to extend that sentence out to include the person doing the action, it would become わたしが日本語を話しました - watashi is the actor, hanasu is the verb, nihonngo is the argument taken by the verb.
There's a concept in Japanese of transitive and intransitive verbs, where this が／を difference becomes incredibly important. Basically you get verbs like "to open" where there are two different ways of using them - i.e. "the door is open" / "he opens the door". In the first one of those examples, the door itself is the object "acting", so it's marked with が in Japanese - in the second, the person opening the door is the actor, so the person is marked が and the door is marked を.
Thing is, In Japanese you also use a slightly different verb in each instance - same kanji, but different pronunciation of the verb stem. This is a fun headache you've got to look forward to
Shinji 5,902 posts
Seen 4 years ago
Registered 15 years ago