From Namiko Abe, Your Guide to Japanese Language.
Particles are probably one of the most difficult and confusing aspects of Japanese sentences. Among particles, the question I am often asked is about the use of “wa（は）” and “ga（が）.” They seems to make many people confused, but don’t be intimidated by them! Let’s have a look at the functions of these particles.
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.
I am a student.
(As for me, I am a student.)
Nihongo wa omoshiroi desu.
Japanese is interesting.
(Speaking of Japanese, it is interesting.)
“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.
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.”
Beside being a topic marker, “wa” is used to show contrast or to emphasize the subject.
Biiru wa nomimasu ga, wain wa nomimasen.
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.
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.
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.
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.
Who is coming?
Yoko ga kimasu.
Yoko is coming.
“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.
Taro went to school.
Taro ga gakkou ni ikimashita.
Taro is the one who went to school.
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.
I want a car.
Nihongo ga wakarimasu.
I understand Japanese.
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.
I didn’t know that Mika got married.