Quick and Dirty Guide to Wa and Ga
Last revised Aug 30, 2000. (Thanks much to Bart-sensei for constructive observations in sci.lang.japan)

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Introduction

Apart from specific patterns for the use of GA presented in the ad hoc section, the difference between those two particules boils down to whether both parties involved in the conversation consider the topic as mutually known. It is known either if it has already been discussed in the past, or is an abstract topic (in which case, it is known "culturally").

The word that marks the topic has to be recognized mutually by both parties as a known concept; Otherwise, if a concept that is purely unknown by the other party is introduced for the first time, it must be marked with GA. This aspect of mutual knowledge is trully important, since it determines whether the speaker uses WA or GA to introduce it.

It helps to think of WA/GA as close to the dichotomy between definite/indefinite articles in English, although you should not rely on this to build Japanese sentences. Were it that easy, there wouldn't be whole books written on the subject... and no threads in ngs :-)

WA

Mutually-known concept

If a concept is already known by both parties the first time it is brought up in the conversation, introduce it with WA. A concept is known if it has already been discussed in previous conversations, or if it is abstract (and, hence, known although it hasn't been introduced in a particular conversation already). The closest equivalent that you can use in English would be "As for <mutually-known concept>, <new piece of infos>."

For example, 男はつらいよ: Here, we're refering to "man" in the abstract, instead of one particular person, so WA is used. Other such examples thrown in for good measure: "人生は一度しかない", "水と安全はただ", "月はウサギ", "春は曙" [from Makura-no-soushi, Seishou-nagon].

Once this topic as been introduced, refrain your urge to utter it in every single sentence to match the SVO pattern that you're used to in English, and just leave it to be understood from the context. A usual example of this error is the "Watashi wa,... . Watashi wa, ..." that foreigners often use with wild abandon. Native speakers do not speak that way.

For instance, if it is obvious that YOU are the one who bought a car, just say "この間、車買った." If, on the other hand, it is someone else who did buy the car and there is no way the listener would know that, it is obviously required that you say so: "この間、香ちゃん（が）車買った". Although the topic would probably be introduced with GA because it's a statement out of the blue, GA is one of those particules that are often left out in colloquial Japanese as long as this does not introduce any ambiguity.

Contrast

Besides its role as topic-marker for a mutually-known concept, WA is used to indicate a contrast between different topics. A classic example of the use of WA to indicate contrast is when used with the negative form, eg. テレビが好きではない. Another example: 僕は、行かなかった (as opposed to someone who did go.)

GA

New topic

A brand new concept, ie. not known by the other party, must be introduced with GA the first time it occurs in the conversation. From then on, the concept will be introduced by WA if need be; If, however, leaving it out altogether does not introduce any ambiguity as to what the topic is, you should just leave it out entirely.

Subject in a predicate

Bart-sensei defines the use of GA as subject-marker in a predicate with "non-deletable non-topic noun in 'subject' position." A predicate is either a stand-alone block of information out of the blue, or additional information about a topic introduced by WA (either explicitely if the topic hasn't been introduced yet, or implicitely if the topic can be understood from the context.) In other words,

[<mutually-known topic> WA,] <subject GA>

Set patterns

Finally, GA is used in a few patterns:

Test your knowledge of "wa" and "ga"

(excerpt from Matsumoto Setsuko and Hoshino Keiko's "日本語能力試験ー１・２級対策 : 文法・語彙編", UNICOM Inc, ISBN 4-89689-137-6, p.139-141).

問題A

  1. 駅の前に新しいスーパー＠できた.
  2. サラリーマン生活＠どう？もう慣れたでしょう？
  3. この赤い花 ＠桜ですか？それとも、あのピンクのはなですか？
  4. "あ！湖＠見える." "ああ、あの湖＠山中湖ですよ".
  5. 道を歩いていたら、知らない人＠私に挨拶をした.
  6. 人の体力＠年をとるとともに、衰えるものだ。最近年のせいか体力＠落ちてきた.
  7. "だれ＠次の社長になるかなあ". "次の社長＠副社長の原田さん＠なるらしいよ".
  8. "黒田さんって、どのひと？" "黒田さん＠あの髪が長くて、きれいな人だよ".

問題B

  1. 息子＠就職した会社は大企業だから、不況にも強いだろう.
  2. 受験生＠筆記試験を受けたあとで面接試験を受ける
  3. あなた＠行かないなら、私も行かないつもりです.
  4. NHK＠行った調査によると、首相の支持率が落ちたらしい.
  5. NHK＠調査を行った後、その結果についての番組を放送した.
  6. 受験戦争＠なければ、もっとのびのびした教育ができるだろう.
  7. 大臣＠南米を訪問したとき、日系人から大歓迎を受けた.
  8. 医学＠進めば進むほど、老人問題も深刻化する.
  9. 川の水＠汚れたせいで、魚が減ってしまった.
  10. 今我々＠解決すべき問題は山積している.

問題C

  1. 漢字の意味＠分からなくて困っています.
  2. 漢字の意味＠わかりますが、読み方＠難しいですね.
  3. もっと大きい車＠欲しいから、積み立て貯金をはじめた.
  4. このところ日中＠暑いけど、朝晩＠ずいぶん涼しくなった.
  5. 君＠金持ちだからいいけれど、私は金がなくて困っている.
  6. 誰＠リーダーシップを握ってみも、政治がよくなることはないだろう.
  7. 電車のドア＠開くと、割れ先に乗り込む乗客は迷惑だ.
  8. 次回のミチング＠いつ＠よろしいでしょうか.
  9. 就職の前に、どこの企業＠自分にもっともふさわしいか、ずいぶん悩みました.

ANSWERS

Mondai A
1.ga 2.wa 3.ga 4.ga, wa 5.ga 6.wa, ga 7.ga, wa, ga 8.wa.

Mondai B
1.ga 2.wa 3.ga 4.ga 5.wa 6.ga 7.wa 8.ga 9.ga 10.ga

Mondai C
1.ga 2.wa, wa 3.ga 4.wa,wa 5.wa 6.ga 7.ga 8.wa,ga 9.ga