Topic vs Subject (は vs が)
は vs が
は marks the topic (what we're talking about), while が marks the subject (who does something) and introduces new information.
Pattern
Topic + は vs Subject + が
Explanation
The difference between は and が is one of the most challenging aspects of Japanese grammar. は marks the topic — the thing already established in conversation: '田中さんは来ました' (As for Tanaka, he came — we're talking about Tanaka). が marks the subject and introduces new information: '田中さんが来ました' (Tanaka came! — this is new information).
A useful test: if you can rephrase the sentence starting with 'As for X...', use は. If you're answering 'Who/What did something?', use が. Question words always use が: 'だれが来ましたか' (Who came?).
Certain predicates always require が: existence (ある/いる), ability (できる/わかる), desire (ほしい/〜たい), and likes/dislikes (好き/嫌い). These describe qualities of the subject, not actions by a topic.
Examples
田中さんは学生です。
たなかさんはがくせいです。
Tanaka-san wa gakusei desu.
Tanaka is a student. (talking about Tanaka)
田中さんが来ました。
たなかさんがきました。
Tanaka-san ga kimashita.
Tanaka came. (new information — answering 'who came?')
私は猫が好きです。
わたしはねこがすきです。
Watashi wa neko ga suki desu.
I like cats. (は marks topic 'I', が marks what is liked)
だれが行きますか。
だれがいきますか。
Dare ga ikimasu ka.
Who will go? (question words always use が)
Common Mistakes
Wrong
だれは来ましたか。
Correct
だれが来ましたか。
Question words (だれ, 何, どこ) always use が, never は.
Wrong
私は日本語は好きです。
Correct
私は日本語が好きです。
With 好き/嫌い, the liked/disliked thing takes が.
Usage Notes
This is a complex topic that even advanced learners continue to refine. At N5 level, focus on the key rules: が for new information/question words, は for established topics, and が with specific predicates (好き, ほしい, わかる, できる, ある/いる).
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