は vs が is infamously hard to pin down when you're trying to learn Japanese. Probably the best you can hope for is to try to accumulate a collection of rules of thumb and let it crystallise as you read more sentences. I'd be a little wary of blasting past the basic grammar thinking "I'll understand this later", but I make an exception here :-)

ボブ：　誰が学生？- Who is the one that is student?
アリス：　ジムが学生。- Jim is the one who is student.

One way to look at this is at the 'rule of thumb' level: (a) question-words never take は and (b) the answer to a question is kind of like handing the questioner his sentence back with the gaps filled in: it uses the same particle the question had. は is OK but not as an answer to the question asked.

The other way is to take a higher level overview and say that は is kind of marking stuff-we-already-know, background-thing-about-which-I'm-going-to-talk, topic. Then from that you can say "it makes sense that you don't use は here because "it's Jim" is the interesting new data we're providing to our questioner, it's definitely not the background already-known thing" and kind of justify your rules of thumb to yourself.

If you can find a grammar book or textbook with some discussion of this it has probably had more thought put into the explanation than you'll get from the average forum post response (including mine!). So you're probably better trying that kind of reference for an initial idea, and then asking questions about specific problems (like "why is this sentence in particular は when I thought xyz..."). If you ask for a general explanation you're likely to end up just getting us all arguing with each other about the detail :-)

In the examples above, the subject (me) is omitted. Usually with body parts or other inherent qualities, instead of 私の背中 (my back, which would sound kind of weird, like your back was something you went out and bought) you would say 私は背中が (as for me, (my) back ...), or if we already know "me" is the subject, just 背中が.

靴が汚れた on the other hand, could technically go either way (私の靴は or (私は)靴が) - not sure which is more common or natural.

Don't think that you use は with things you're already talking about, が with things you're introducing, etc. etc.  That's a popular but wrong explanation.  Furthermore, は doesn't mark "subject"--it can just as easily go with the object, so don't say that either.

If you've heard those explanations, please try to forget them.

A large part of this problem stems from people not grasping the topic/comment relationship in the pure form in which it exists in Japanese.  English grammar muddies the issue by combining topic and subject into one position--this works in most of the cases when topic and subject are the same, but when they're not, you have to shuffle the rest of the sentence around to end up with the topic in subject position.

(One particularly mind-bending type of shuffling is called "subject raising."  Linguists like to argue about whether it happens at all in Japanese.  On the other hand, if you try to invent a logically precise language with positional grammar and no topic provision, it can really ruin your day.  Hmm, I think I'll take the Japanese grammar.)

So, topic/comment.  When you make a statement about something you're often connecting two things: a concept shared between you and the listener (topic) and a statement that further describes that concept (comment). 

In questions, the comment part can have blanks (marked with words like 誰、どれ、どこ、いつ) or ask the listener to agree or disagree with the comment (intonation, か、かい); however, the topic cannot have blanks.

If the topic is already obvious to both the speaker and the listener, it is completely dropped: no need to state the obvious.

If you don't think you listener will know the topic, you have to spell it out.  You can give examples with 「や」--this seems to be the only one that doesn't confuse English speakers, so I'll ignore it

You can specify the topic with 「は」.  This automatically gives something of a contrastive feel, because it says "of all the possible topics, I pick this one."

You can add a topic with 「も」.  This particle is a tricky pain in the ass.  Seriously.  You think は／が is bad?  Well, this one means "there may be many possible topics for the following comment and I'm not going to say what they are, but this marked topic definitely goes with this comment.  So there."  It's usually introduced as meaning "also," but that's not it's core meaning: it can appear alone.

So, that's the topic part.  Now, the comment.  が and を, the identifier particles, are most often part of a comment.  Comments are usually structured around something acting as a verb: either a real verb or an adjective/noun describing something--in any case it always ends up at the end.  But, it's often not enough to just say "did."  You have to identify who did, or what is pitch black, or what you ate yesterday.  That's where が and を come in.  が identifies subjects.  を identifies を-things, usually what you'd call "direct objects" in English.  Exactly what those terms mean depend on the verb and verb form; think of them as two slots that the verb assigns meaning to.

For example, 食べる means "organism (が) eats food (を)." 

「食べた」<= comment that something ate or something was eaten.
「ケーキを食べた」<= like above, except ケーキ is identified as being eaten.
「ケーキが食べた」<= like above, except ケーキ is identified as having ate something.

Where this gets tricky is that "identification" is really just a way to further elaborate a verb, not a way to elaborate a noun.

For example,
「車だ」"is car"
「赤い車だ」"is red car"
In the second comment, we know more about the car because the adjective 赤い describes it.  We're not saying something about redness, we're not saying something about car, (by the definition of "topic") we're saying something about the topic, whatever that is.

When you add a が or を to a verb, we know more about how the verb happened.

Consider
A -「猫はどこ？」
B -「食べている。」

The first sentence identifies a topic: "cat", picking it out of the cloud of possible topics, then has a fill-in-the-blank question about place "where?"
The second question has an obvious topic (thus unstated) and offers the comment "is eating."

If we replace the second sentence with:
B - 「ケーキが食べている。」
we elaborate what's happening.  "Where's the cat?" / "The cake is eating it."  Poor kitty.

B - 「ケーキを食べている。」 is perhaps a more politically correct replacement.  Now, the cat is eating the cake. 

B -「お前が食べている」is left at as an exercise for the reader, but note that

A -「猫はどこ？」
B -「お前は食べている。」 is a non-sequitur: it leaves the question unanswered and raises a new topic.

Hopefully, this is enough to get you started, since it's time for me to go to bed.  Here's what you have to do to extend your knowledge:

Continue reading.  Take some sentences, pick them apart, find the topic and comment.  Notice how answers to questions don't usually change the topic--if they do, try to understand why.

For later: changing topic when contrasting two things: "contrastive は".  も vs English "also" and "even."

Last edited by wildweathel (2009 December 28, 11:09 pm)

I don't see anything wrong with your examples.  I'm trying to preempt the less-than-right-or-helpful descriptions that everyone's googled.  No offense taken, I hope?

AAP is correct and a kick-ass resource.  Only problem I have with it is that it usually stays on the surface and doesn't dig down to the core things.  It'll give you a dozen uses of が, all of them correct and well explained, and not give you the common thread between them (elaborating a verb by identifying its subject).

(hmm, that description of が with nouns/adjectives/state-verbs falls flat.  I'll need to think it over.)

I should also remind everyone that I'm just some loudmouth brat on the Internet, please argue with me, especially if it gets you thinking and looking to understand things better yourself.

As for about.com, it's full of fuzzy cop-outs like "indicates emphasis" and "depends on the context or the intonation"--especially unhelpful when they tell you は means emphasis and then that が means emphasis, too!  The only solid thing it says that I disagree with is that が indicates new information.  が correlates with new information because new information has to go in the comment, and thus can't be marked with は.  You can have が as part of a topic, where it necessarily goes with "old information":

ビルが高い都市には　(topic)
小さなアリのような人が住む。(comment)

In the city where buildings are high (topic)
live people like small ants. (comment)

Last edited by wildweathel (2009 December 28, 11:08 pm)

Oh noes, another ha/ga thread.

guffaw!

I was actually thinking that ha/ga deserved a meme picture when I made that post.

Last edited by Jarvik7 (2009 December 29, 8:04 am)