Topic は vs Subject が

は and が are the two particles that most often land on the same noun, and choosing between them trips up learners for years. The は vs が particle page attacks the choice from the angle of information status (old information → は, new information → が). This page attacks it from the angle of sentence structure: は marks a topic, which is a discourse notion, while が marks a subject, which is a grammatical relation. They are not two flavours of "subject marker" competing for one slot — they are different kinds of thing operating on different tiers, which is exactly why they can appear in the same sentence.

Two different jobs

• は marks the topic — what the sentence is about. It is a discourse-level flag ("as for X…"). A topic is old, given, presupposed; the new information lives in the comment that follows.
• が marks the subject — who or what performs or undergoes the predicate. It is a clause-level grammatical role. It often carries new, identifying information; the new information lives on が itself.

田中さんは sets up a known person and comments on him — the news is "is a teacher." 雨が降っている, by contrast, reports a whole fresh scene: rain is the new subject filling the subject slot of 降っている, and nothing is presupposed. That difference — comment on a known frame vs report a new event — is the difference between topic and subject.

The answer test

The most reliable structural probe is to ask what question the sentence answers. Where the new information sits tells you which particle to use.

• が answers "who?" / "what?" — the spotlight is on the subject.
• は answers "what about it?" — the spotlight is on the predicate.

Take the minimal pair everyone should memorize:

私が行きます answers "who's going?" — it singles out the speaker as the subject, and everyone else is implicitly not going. 私は行きます answers "what about you — are you going?" — the person is the known topic and "will go" is the news. Same three words minus the particle; opposite information structure. This shows up crisply in question-and-answer pairs:

The question word 誰 is the missing information, so it takes が; the answer 田中さん supplies that information, so it keeps が. Try to topicalize either with は and the sentence breaks — you cannot frame a discussion around the very thing nobody knows yet. (The old/new logic behind this is drilled on the particle page.)

が is a case particle; は is not

Here is the structural fact that proves は and が are different categories, not two subject markers. が is a case particle — it grammatically marks the subject argument of a clause, and it lives inside the clause's syntax. は is a binding (information) particle — it floats above the clause and flags a topic, and it can attach to almost anything.

Three consequences follow, and they are testable:

1. Only が can be the subject inside a relative or subordinate clause. A plain topic は is banned there, because a subordinate clause is not "what the whole sentence is about."

Inside the relative clause 母が作った ("[that] my mom made"), the subject must be 母が. You cannot say ×母は作った料理 as a plain relative clause. Meanwhile the whole topic — 料理 — takes は because that is what the sentence is about. When you want to flip the spotlight onto that embedded subject and make it the new information, Japanese has a dedicated focusing device — the のは cleft construction.

2. は can attach to a non-subject; が cannot. Because は only claims aboutness, it happily sits on an object or an adjunct. が is welded to the subject role.

Here この映画 is the object of 見た, topicalized with は. There is no way to do this with が, because が would force 映画 to be the watcher.

3. They coexist, on different tiers. The clinching evidence — one clause, both particles, no conflict:

象は is the discourse topic (the whole animal); 鼻が is the grammatical subject of the inner statement 鼻が長い ("the nose is long"). Two tiers, two particles, zero competition. If は were a subject marker, this sentence would be impossible.

Fresh observations always take が

One practical corner worth isolating: when you notice something and report it on the spot — a sudden perception with no pre-established topic — the whole thing is new, so it takes が (this is the "neutral description" reading of が).

There is no "as for the bus…" here; you are announcing a fresh event whole. Reach for は in these and the sentence sounds like you are contrasting the bus with something else, which is not what a spontaneous exclamation means.

Common mistakes

Mistake 1 — Defaulting to は because it "feels like the subject." For a fresh observation, this flattens the structure.

Mistake 2 — Answering a "who?" question with は. The reply supplies new information, which must ride on が.

Mistake 3 — Putting は inside a relative clause. The embedded subject must be が.

Mistake 4 — Using が for a plain, known topic. が turns a neutral statement into an exclusive "it's ME who…".

Key takeaways

• は = topic (a discourse flag, "as for X"); が = subject (a clause-level grammatical role).
• The answer test: が answers "who/what?" (spotlight on the subject); は answers "what about it?" (spotlight on the predicate).
• が is a case particle — it lives inside relative and subordinate clauses, where a plain topic は is banned.
• は attaches to non-subjects (objects, adjuncts) and floats above the clause; が is welded to the subject.
• They coexist (象は鼻が長い) precisely because they operate on different tiers — proof that は is not a subject marker.