Also, on a different note, I was reading through the thread and noticed the discussion about は, が, and を several pages back, particularly regarding the difference between the first two. However, all three are interrelated so it helps to cover all three at once.
The difference between these three is that は (wa) is a topic marker, which can (in conjunction with other particles, where appropriate), mark any part of speech where that item is the topic of the sentence. Thus, it covers both subjects and objects in the grammatical sense. In this capacity, it also marks the topic as something which neither excludes other possibilities (where ga, o, or another more specific particle would be used), nor exists in addition to another topic (where one would use mo as a way of linking them up). Hopefully that should become a little clearer below…
が (ga) marks the grammatical subject of an action or statement, usually such that the subject also becomes emphasized. To illustrate, consider this exchange:
A: Karen-san wa keeki o tabeta ka?
B: Iya, Jimu-san ga keeki o tabeta.
A wants to know if Karen had any cake, but B states in no uncertain terms that Jim was the one who ate it (and implying that Karen, who may or may not be speaker B, did not).
On the other hand, if B replies:
B: Karen-san wa tabeta kedo…
B indicates that Karen (who in this case is definitely not speaker A or B) did have cake (thus keeping her as the topic), but that the action is not necessarily exclusive to Karen. Most likely, someone else also had cake (or perhaps someone else who is about to be named was not able to have any because Karen was a pig and ate the whole thing).
Similarly, を (o) marks the grammatical object of an action only. Thus, here:
A: Keeki wa tabeta?
B: Iya, Aisukuriimu wo tabeta.
A wants to know if B had any cake (with cake being the topic), but B replies that he had the ice cream, not the cake (the o both marks the ice cream as the object and sets it in exclusion to the cake). However, if B answers:
B: Keeki wa tabeta kedo…
B is saying that he did have cake, but he probably had other things as well (or at least wanted to have other things, but wasn't able to). In the world of grammatical imagination, B could also conceivably reply:
B: Iya, keeki ga tabeta.
However, this would imply that the cake has somehow come to life and is devouring the other foodstuffs / party guests, so you're relatively unlikely to encounter this scenario.
So, uh… I hope that wasn't too confusing. I know it's a lot of ground to cover, but these are three of the most important particles you need to know when it comes to deciphering a sentence (especially when things get really nonreferential and abstract). So... good luck? ^^;