日本語 Japanese Ask Questions Thread 4

If you have a question about the language, ask it and fellow 4channers might see it and answer it for you.


Japanese Ask Questions Thread2
http://dis.4chan.org/read/lang/1206158123/1-40

Japanese Ask Questions Thread3
http://dis.4chan.org/read/lang/1267485093/1-40

Would this sentence be correct?

誰が日本語がわかりますか。
The intended translation being:
"Who understands Japanese?"

If I break apart the sentence, we could extract it like:
誰が "As for (who)" わかります "understands" 日本語が "Japanese" か　"Question"

That leads me to the next question, can anyone recommend any good resources (internet, books) for learning how to distinguish between が and は?  Initially I thought that が was for new topics, while は is for topics that are already known, but now I understand that isn't always true.