Wa vs ga: the difference, finally clear
It's THE question every learner asks: when do you use は and when do you use が? Both seem to mark "the subject", but they play very different roles. Once the logic clicks, the choice becomes almost mechanical.
は frames what's already known, が points at the new information. Everything else follows from that single rule.
は (wa)
Sets the frame: "as for X". The topic is already known to both speakers, the interesting information comes after it.
が (ga)
Puts the spotlight on the word before it: that word is the new information. What comes before が is exactly what you wanted to know.
The minimal pair
Take the same sentence with each of the two particles. Both are grammatically correct, but they don't answer the same question.
私は学生です。
わたし wa がくせい です。
I am a student.
Natural answer to "so, what do you do?". The topic 私 (わたし) is known, the new information is 学生 (がくせい).
私が学生です。
わたし ga がくせい です。
I am the student.
Answer to "who is the student?". が spotlights 私 (わたし): me, not someone else.
Question words demand が
誰 (だれ, who), 何 (なに, what) or どれ (which one) can never be topics: by definition, the answer isn't known yet. So they always take が, and the answer keeps the same が.
誰が来ましたか。
だれ ga きました か。
Who came?
And the answer keeps が: 田中さんが来ました (たなかさん ga きました). Never は here.
First mention, then follow-up
In a story, an element enters the scene with が (it's new), then becomes the topic with は (it's now known). It's the same mechanism as "a dog… the dog" in English.
公園に犬がいる。その犬は大きい。
こうえん ni いぬ ga いる。その いぬ wa おおきい。
There's a dog in the park. That dog is big.
犬 (いぬ) enters the scene with が, then gets picked back up with は.
The quick test
Facing a sentence, run through these questions in order. The first one that matches gives you the particle.
- Is the subject a question word, or the direct answer to a "who / what / which" question? → が.
- Is the element appearing in the story for the first time? → が.
- Are you talking about something already established, known to both speakers? → は.
- Introducing yourself or describing a general frame? → は by default.
One last reflex: some verbs and adjectives like 好き (すき, to like), 分かる (わかる, to understand) or できる (can do) demand が on their complement. The full list lives in the dedicated が article.
Go deeperは (wa): the topic particle, explainedHow the particle は (wa) works, why it's pronounced "wa" and not "ha", and how to use it in a real Japanese sentence.Go deeperが (ga): subject marker and the wa vs ga trapHow が marks the grammatical subject, why it's mandatory with certain verbs (好き, ある, 分かる), and how to choose between は and が.