"I already finished my homework."
Well, in the English version, "I" is the subject and "homework" is the direct object. Either one or neither could be the topic. Without the rest of the conversation, we really can't say. In the Japanese version, "homework" has been explicitly marked as the topic of discussion. The subject could be "I" or it could be "Bob" or anybody else. Without the rest of the conversation, we can't tell. All we know is that it must be the object, because the homework didn't do anything, so it can't be the subject of "did."
even worse haha, in this case you would want は or を, but not が. it's because the homework is getting finished, it's the object - but wa works fine though too if the speaker is bringing up the topic of homework in conversation, trying to show contrast (ex. "did you do the dishes?" "no... but i already did my homework!").