Differentiating が or は(pronounced wa) can't be defined by rules. They aren't the same kind of particle, and がは and はが are consequently impossible. Particles show function, so focus on this more than anything.
が is specific. The subject is identified and the statement (predicate) tells something new about it. が may mark a person/thing carrying something out as a phenomenon occurs. The action is not habitual with が. It may also reveal the existence of something.
Literally: Your head is good.
The waves are high.
It is the sky which is blue.
Nuance Note: が may particularly point something out as the focus of one's statement, and in doing so, a small pause typically follows が. Commas aren't necessary for this, but you may see them used here.
Many adjectives are typically used with が because they show new information.
4. コンピューターがほしいです。 （Polite)
I want a computer.
I like manga.
Part of Speech Note: "To want", "to like", etc. are adjectives in Japanese.
New information does not equate to contrast. So, it's not that the sentence below equates to either the bird sings or the bird sings. Providing details is something we do all the time in a conversation, but it's not the case that we make each detail a topic of discussion. If we do, we return later and address them as topics of a new paragraph or statement (if you're talking). が does have an emphatic usage, but the contexts are not the same. We will see this soon.
The bird(s) sings.
Sense of Discovery
が may show what's found out, whether it be the beginning of a story, discovery itself, etc. As such, with question words you seek out the unknown.
Ah, this is snow!
Conversations have a topic, which is somewhat familiar to the speaker and listener(s). は doesn't show new information. So, "AはBだ" shows something of conversational value. To better show how this differs with "AがBだ", compare the following.
9. リンゴが赤い。 10. リンゴは赤い。
The apple is red. Apples are red/an apple is red/as for apples, they are red.
Kim Jeong Eun is a/the bad person.
Kim Jeong Eun is a bad person.
In 11, the listener(s) may not know about Kim Jeong Eun or about this "bad" quality. In 12, both the speaker and listener(s) know the topic well and are probably discussing North Korea.
は and が are called the topic and subject markers respectively with the understanding that if either is absent, the topic and subject may coincide with each other. If は were replaced by が, が would change the topic into the focus. Focus doesn't mean contrasting with anything. Rather, it draws attention to a statement, and this is in line with everything we've seen about が thus far. Try not to dwell on the names of these particles. Topic and subject don't help us a whole lot in using these particles perfectly 100% of the time, but they do help us grasp them well for the meantime.
Intonation Note: There is normally a small pause that accompanies は when showing a topic.
The Sea of Japan is wide.
This fish is small.
Tea for me.
Meaning Note: This sentence doesn't mean "I am tea". は doesn't specifically link the subject with the verb.
Topic は Subject が X
X is an adjective or verb, and the topic and subject can flip depending on importance. Typically, though, this is the most natural ordering. This pattern is used to discuss a matter concerning the topic.
As for elephants, their noses are long.
Japan has many shrines.
Expressing Nationality and Occupation
は is used when expressing nationality or occupation.
|Nationalities||Occupations (Plain)|| Occupations (Polite)|
| フランス人 ＝ French person|| 教師（きょうし） ＝ Teacher|| 先生 ＝ Teacher|
| アメリカ人 ＝ American person||学生 ＝ Student|| 学生さん ＝ Student |
|イギリス人 ＝ English person|| 医者（いしゃ） ＝ Doctor||お医者さん ＝ Doctor|
|中国人 ＝ Chinese person||銀行員（ぎんこういん） ＝ Banker||銀行員さん ＝ Banker|
Note: The polite occupations aren't used in reference to oneself.
She is from Tokyo.
She is Chinese.
She is a teacher.
23. 「だれが学生ですか」「私が学生です」 (Polite)
"Who is a student? ”I am a student".
Questions (1):Translate the following.
1. I'm not Japanese. (Polite)
2. I am Chinese.
3. Yes, the book is difficult.
Part II: Fill in the appropriate particles.
There are three apples. That's a red apple.
That person (over there) is a student.
は may highlight the scope of a sentence by indicating contrast and can do so even when there aren't two things stated. In a negative sentence it emphasizes the negation. は can be seen after other things such as other particles or adverbs.
I can hear the dog, but I can't see the dog.
Grammar Note: While the first は is thematic, the second shows contrast.
I won't go. (Giving the emphasis that you aren't and probably don't want to.)
There is no ballpoint pen.
I always go.
Sentence Note: This sentence implies that you aren't going this time.
I'll pay the money.
Questions (2): Translate the following.
1. I like dogs (but I don't like ...). 2. She is not a doctor. 3. She is not the doctor.
More が VS は
A lot can be said about the difference between が and は. So, let's look at more minute differences.
What is problematic?
は is used with a question word (interrogative) when used as a predicate. So, は will come first. You cannot attach は to a question word in a question statement. Why? Its purpose is not to seek new information. That's が’s role. So, when you phrase with "something + は + question word ?", you're emphasizing what you want known, not putting focus on the question word.
What is the problem?
Literally: The problem is what?
Your name is (what)?
Grammar Note: In set phrases like "何はなくとも" meaning "if nothing else", は doesn't show that the question word is seeking new information. So, it is OK.
Who is what, and what is where.
Since は shows the topic, it shouldn't be in participial (adjectival) phrases that modify a noun phrase. We know we can attach verb/adjective phrases no matter how long before a noun, but this is the first time we've seen any particle restriction on this. If there weren't any, how would we know where one starts and separate it from the part of the sentence before it (if there is anything)? This difference between は and が helps us do just that.
To read the book that Suzuki brought.
Suzuki reads a book that he brought.
1. Which is wrong? 誰は本を読む？or 誰が本を読む？
2. What is the difference between あなたがすきな本を読む and あなたはすきな本を読む?
3. What is the difference between 犬は歩く and 犬が歩く?
You may use 漢字 at your discretion. Different (polite) variants are not listed, but they would still be right.
4. が; は
1. 犬は好きだ。 2. 彼女は医者じゃない。 3. 彼女がお医者さんじゃない。
1. 誰は本をよむ？ X
2. In the first sentence, the speaker reads the book that 'you' like. The second sentence means "you read a book that you like".
3. 犬は歩く ＝ dogs walk. It's a general statement. 犬が歩く ＝ The dog(s) walk. This is not a known fact when said.