Ga marks what the Japanese call the grammatical subject of the sentence. Think of the subject in the following two ways:
First in neutral descriptions of observable actions or situations.
手紙が来ました。 てがみがきました。 The mail came. Tegami ga kimashita.
雨がふっています。 あめがふっています。 Rain is falling. Ame ga futte imasu.
*However, が cannot be used in the negative form (because, they are in a sense, unobservable). は is used instead.
手紙は来ませんでした。 てがみはきませんでした。 The mail didn't come. Tegami ha kimasen deshita.
雨はふっていません。 あめはふっていません。 Rain isn't falling. Ame ha futte imasen.
Second, for special emphasis, to distinguish a particular person or thing from all others.
私がしました。 わたしがしました。 I did it. Watashi ga shimashita.
ハームバーガが緑です。 ハームバーガがみどりです。 (The) Hamburger is green. Haamubaaga ga midori desu.
Other Uses. When and interrogative pronoun is used as the subject of a sentence, が must be used.
誰が,これをしましたか? だれが,これをしましたか? Who did this? Dare ga, kore wo shimashita ka?
Rinji_HalfElf
Other notes
*As an abstract and rough approximation, the difference between は and が is a matter of what the focus of the sentence is.
は gives focus to the action of the sentence, i.e., to the verb or adjective.
が gives focus to the subject of the action.
An example of this can be illustrated with two sentences which mean almost the same thing in English.
(あなたは)日本語を話しますか? (あなたは)にほんごをはなしますか? Do (you) speak Japanese? (anata ha) nihongo wo hanashimasu ka?
(あなたは)日本語が話せますか? (あなたは)にほんごがはなせますか? Can (you) speak Japanese? (anata ha) nihongo ga hanasemasu ka?
Exhaustive ga
*Unlike は, the subject particle が nominates its referent as the sole satisfier of the predicate. This distinction is famously illustrated by the following pair of sentences.
ジョンは学生です。 ジョンはがくせいです。 John ha gakusei desu John is a student. (There may be other students among the people we're talking about.)
ジョンが学生です。ジョンががくせいです。 John ga gakusei desu. (Of all the people we are talking about) it is John who is the student
Rinji_HalfElf
Transitive and Intransitive verbs and が
Japanese has a large variety of related pairs of transitive verbs (that take a direct object) and intransitive verbs (that do not take a direct object), such as hajimaru, (an activity) begins and hajimeru, (an actor) begins (an activity).
授業が始まる。 じゅぎょうはじまる。 Jugyou ga hajimaru. The class starts(intransitive). (It is the class which does the starting on it's own, therefore it's observable, like rain.)
先生が授業を始める。せんせいが授業を始める。 Sensei ga jugyou o hajimeru. The teacher starts(transitive) the class. (Of all the teachers, this one starts class.)
Then there's another variant: 先生は授業を始める。せんせいは授業を始める。 Sensei ha jugyou o hajimeru. The teacher(s) start(s)(transitive) the class. (Teachers in general start the class.)
And another variant: 先生のみは授業を始める。せんせいのみは授業を始める。 Sensei nomiha jugyou o hajimeru. The teacher(s) start(s)(transitive) the class. (Only teachers in general start the class. Sounds silly, but many times this is understood as a general statement in English without the extra words.)
Yet another example: 先生のみが授業を始める。せんせいのみが授業を始める。 Sensei nomiga jugyou o hajimeru. The teacher starts(transitive) the class. (Out of all the teachers, this one starts the class only.)
And if nomi "のみ" is moved next to the verb as "のみ始める", it might indicate that no one else starts the class but this teacher (and not you).
In many cases the subject is omitted. Had the "teacher" been the topic of conversation in the first transitive example, the following would occur:
(先生が) 授業を始める。(せんせいが)授業を始める。 (Sensei ga) Jugyou o hajimeru. Class starts(transitive). (By an actor already mentioned, as indicated by the transitive verb or previous context)
The omitted subject is also what makes this topic more difficult to explain. Many examples used do not include the omitted subject or previous context which is important to understanding for many learners. Those who speak English and begin to learn Japanese see most articles on this subject as a list of grammatical memorizations because they are not proficient with omitted subject. Even if a student knows how to drop a subject, they also must learn to when insert one for some random examples to make sense. Transitive and intransitive makes this a little clearer.
Rinji_HalfElf
http://en.wikipedia.org/wiki/Japanese_grammar
Rinji_HalfElf
However, would TV-wo kau-koto-ga dekiru be acceptable? In the case of -ga, it marks the object when a verb is in the potential mood, as if the verb was passive...
Zhen Lin
テレビを買うことができる is acceptable.
bamboo4
The book Making Sense of Japanese: What the Textbooks Don't Tell You by Jay Rubin offers an excellent section explaining the difference between が and は.
anonymous
Need to put together something on when to choose to use が (subject) instead of は (topic)
Amatuka
Note that が (or の) replaces を in sub-sentences. (See 'television' example, would be テレビを買う (terebi wo kau) if it was a sentence on it's own.
Amatuka
が [ga] follows a noun or a noun phrase. (e.g. 犬が [inu ha] 'the dog' / 'dog(s)' 知るのが [shiru no ga] 'what (I) know (is)' The latter case の nominalizes (turns into a noun) 知る)
N = noun
Amatuka
In #880, テレビが買うことができる is not acceptable.
bamboo4
Could you explain what you leant from the book the difference between が and は?
dakara?
You can read Tae Kim's online guide to japanese and get a very reasonable explanation. First of all he rejects the use of the term subject, simply because it is laden with all the wrong connotations from english. Primarily because subject in japanese is not the same as subject in english (and most roman and germanic languages, probably). He calls it the identifier, a particle used to identfy one among many, where as ha is used to say something detailed about something, by some called the focus particle.
Svendsen
@Amatuka テレビが買うことができる is wrong @bamboo4 Yes, テレビを買うことができる is correct
Amatuka is most likely confusing を with が in clauses: if you want to say 背が高い友達, you can also say 背の高い友達. There is no difference at all (even in connotation) between の and が as far as every single Japanese person I've ever asked about this is concerned, when dealing with this type of clause.
KyleGoetz
Can anybody tell me difference between は and が. As both are topic markers, i dont understand when to use which
devricha
Can anybody tell me difference between は and が. As both are topic markers, i dont understand when to use which
devricha
if i'm understand right, a good comparison would be 私は学生です.I am a student. Because here ha focuses on me in particular. it's a narrower description.
学生がいます.There is a student. because this has a broader sense, there are many students but here is one.
feel free to correct me but that is what i've come to understand. and thanks to svendsen because i got most of it from his comment.
nellyaudrey
"ha" is used when introducing/describing something
while "ga" is used when you want to be specific and direct the focus of attention to the noun preceding "ga"
more or less the difference lies on the weight of "emphasis" ~ tada hitori no iken desukedo ^w^
takamichi
@nellyaudrey,I would say が introduces new subject in sentences like 学生がいます, so it's new information for the listener/reader - "there is a student",when 学生は would sounds like we already known him or he has been already introduced - "the student...".Thus if say something like "There was a student in the class.The student was very clever" in Japanese it would be 学生が in the first sentence and 学生は in the second one,imo.
avidlearner
There are 3 が's: 1. Subject marker, can only attach to nouns, but in informal speech sometimes the nominalizing の is dropped - can distinguish from the second use based on context. 2. Contrastive conjunction. Connects two clauses, basically just means but. 3. Possessive. Can view this as a more limited version of の, where it only marks possessive and doesn't nominalize. See 我が国,我がまま, and sentences like 私が好きな食べ物はラーメンだ, and notice that there is literally no difference structurally if you were to switch the が for a の. The subject of that sentence is 食べ物, but は masks the subject in cases where the subject and topic are the same.