Topic Marker and Subject Marker
Roughly speaking, "wa" is a topic marker, and "ga" is a subject marker. The topic is often the same as the subject, but not necessary. The topic can be anything that a speaker wants to talk about (It can be an object, location or any other grammatical element). In this sense, it is similar to the English expressions, "As for ~" or "Speaking of ~."
| Watashi wa gakusei desu. 私は学生です。 | I am a student. (As for me, I am a student.) |
| Nihongo wa omoshiroi desu. 日本語は面白いです。 | Japanese is interesting. (Speaking of Japanese, it is interesting.) |
Basic Differences Between Ga and Wa
"Wa" is used to mark something that has already been introduced into the conversation, or is familiar with both a speaker and a listener. (proper nouns, genetic names etc.) "Ga" is used when a situation or happening is just noticed or newly introduced. See the following example.
| Mukashi mukashi, ojii-san ga sunde imashita. Ojii-san wa totemo shinsetsu deshita. 昔々、おじいさんが住んでいました。 おじいさんはとても親切でした。 | Once upon a time, there lived an old man. He was very kind. |
In the first sentence, "ojii-san" is introduced for the first time. It is the subject, not the topic. The second sentence describes about "ojii-san" that is previously mentioned. "Ojii-san" is now the topic, and is marked with "wa" instead of "ga."
Wa as Contrast
Beside being a topic marker, "wa" is used to show contrast or to emphasize the subject.
| Biiru wa nomimasu ga, wain wa nomimasen. ビールは飲みますが、 ワインは飲みません。 | I drink beer, but I don't drink wine. |
The thing being contrasted may or may not stated, but with this usage, the contrast is implied.
| Ano hon wa yomimasen deshita. あの本は読みませんでした。 | I didn't read that book (though I read this one). |
Particles such as "ni（に）," "de（で）," "kara（から）" and "made（まで）" can be combined with "wa" (double particles) to show contrast.
| Osaka ni wa ikimashita ga, Kyoto ni wa ikimasen deshita. 大阪には行きましたが、 京都には行きませんでした。 | I went to Osaka, but I didn't go to Kyoto. |
| Koko de wa tabako o suwanaide kudasai. ここではタバコを 吸わないでください。 | Please don't smoke here (but you may smoke there). |
Whether "wa" indicates a topic or a contrast, it depends on the context or the intonation.
Ga with Question Words
When a question word such as "who" and "what" is the subject of a sentence, it is always followed by "ga," never by "wa." To answer the question, it also has to be followed by "ga."
| Dare ga kimasu ka. 誰が来ますか。 | Who is coming? |
| Yoko ga kimasu. 陽子が来ます。 | Yoko is coming. |
Ga as Emphasis
"Ga" is used for emphasis, to distinguish a person or thing from all others. If a topic is marked with "wa," the comment is the most important part of the sentence. On the other hand, if a subject is marked with "ga," the subject is the most important part of the sentence. In English, these differences are sometimes expressed in tone of voice. Compare these sentences.
| Taro wa gakkou ni ikimashita. 太郎は学校に行きました。 | Taro went to school. |
| Taro ga gakkou ni ikimashita. 太郎が学校に行きました。 | Taro is the one who went to school. |
Ga in a Special Circumstance
The object of the sentence is usually marked by the particle "o," but some verbs and adjectives (expressing like/dislike, desire, potential, necessity, fear, envy etc.) take "ga" instead of "o."
| Kuruma ga hoshii desu. 車が欲しいです。 | I want a car. |
| Nihongo ga wakarimasu. 日本語が分かります。 | I understand Japanese. |
Ga in Subordinate Clauses
The subject of a subordinate clause normally takes "ga" to show that the subjects of the subordinate and main clauses are different.
| Watashi wa Mika ga kekkon shita koto o shiranakatta. 私は美香が結婚した ことを知らなかった。 | I didn't know that Mika got married. |
Review
Now let's review the rules about "wa" and "ga."
| wa は | ga が |
| * Topic marker * Contrast | * Subject marker * With question words * Emphasize * Instead of "o" * In subordinate clauses |
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Source: http://japanese.about.com/library/weekly/aa051301a.htm
