は vs が

How it works

は marks the topic: the thing already on the table that the rest of the sentence comments on. Read it as ‘as for …’ or ‘speaking of …’. が marks the subject as new or singled-out information: it answers ‘who?’ or ‘what?’ and picks one item out of the possible candidates.

Three rules cover most N5 sentences:

1. Known information takes は, new information takes が. 田中さんは学生です tells us something about Tanaka; 田中さんが学生です answers ‘Who is the student?’.
2. Question words and their answers take が. だれが来ましたか。→ 田中さんが来ました。 Never だれは.
3. Predicates of liking, ability, wanting and existence mark the thing liked, understood, wanted or existing with が. 好き・きらい・分かる・できる・ほしい・上手・ある・いる: 寿司が好きです, 日本語が分かります, 時間がありません. That noun is the grammatical subject, not an object, so を is out.

A sentence can carry both: 象は鼻が長いです. は sets the frame (elephants), が names what inside that frame is long (the trunk). Neutral reports of something you just noticed (rain, a noise, a cat) take が: 雨が降っています.

は also carries contrast. コーヒーは飲みますが、お茶は飲みません sets coffee against tea, and 魚は食べません, with は in place of を, means ‘fish I don’t eat’ (other things, yes).

Examples

• 私は学生です。（わたし は がくせい です）— I am a student.
• だれが来ましたか。（だれ が きました か）— Who came?
• 田中さんが来ました。（たなかさん が きました）— Tanaka came.
• 私は寿司が好きです。（わたし は すし が すき です）— I like sushi.
• 象は鼻が長いです。（ぞう は はな が ながい です）— Elephants have long trunks.
• 雨が降っています。（あめ が ふって います）— It is raining.

Compare with

が with 好き・分かる・できる・ほしい・あります・います

These predicates work like Spanish gustar: the thing liked, understood, possible, wanted or existing is the grammatical subject and takes が, not を. 車がほしいです, 漢字が読めます, 猫がいます.

も

も replaces both は and が: 私も学生です, 田中さんも来ました. They never stack (no はも, no がも).

Contrastive は

A second は in a sentence, or は after an object or a place, means contrast: 肉は食べますが、魚は食べません. Like も, は replaces を and が (肉は, not 肉をは) but stacks after に and で: 東京には, ここでは.

Traps for Spanish, English and French speakers

• English ‘I like sushi’ has a direct object, so English speakers write 寿司を好きです instead of 寿司が好きです. Spanish speakers can lean on ‘me gusta el sushi’: el sushi is the grammatical subject there too, and the subject of 好き・分かる・できる・ほしい takes が.
• Spanish ‘¿Quién vino?’ and French ‘Qui est venu ?’ open with the question word as subject, and learners copy it as だれは来ましたか. Question words never take は, so it is だれが来ましたか, and the answer keeps が: 田中さんが来ました.
• English and French need a subject in every sentence, so English and French speakers start everything with 私は, which after the first time sounds contrastive, like ‘as for ME…’. Spanish speakers already drop ‘yo’ when the verb makes it clear: do the same in Japanese and say 学生です, 明日行きます.
• Textbooks call が ‘the subject marker’, so learners mark every subject with it and introduce themselves with 私が学生です or report the weather as 今日が寒いです, which single something out (‘I am the one who is a student’, ‘today is the cold day’). A plain statement about a known frame (me, today, this shop) takes は: 私は学生です, 今日は寒いです.