Notes: Particle "Wa" & "Ga"

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