は vs が: New vs Known Information

This page opens the Choosing group, and it does something the particle pages deliberately don't: it hands you a decision procedure. The は vs が particle page and the topic-vs-subject page teach the whole は/が system from the ground up — the double-subject sentence, the case-particle asymmetry, all of it. Here we zoom in on one blade of that system, the single most useful one for a beginner: information status. Give me one question about a sentence and I will give you the particle, correctly, most of the time.

The one question

Before you pick は or が, ask yourself exactly this:

Does the listener already have this piece of information, or am I supplying it now?

• Already has it (old, known, in view) → は
• Supplying it now (new, just introduced, the answer to an unspoken "what/who?") → が

That is the entire procedure. English has no particle for this, so there is nothing to transfer — an English subject is just a subject, whether it's old news or a bombshell. Japanese sorts subjects by their newness, and once you start hearing that, the coin-flip stops.

Same three-word skeleton, opposite particle. 猫がいる reports a fresh discovery — the cat is brand-new information, so が. 猫はいる takes a cat everyone already has in mind and comments on it ("as for the cat — it's here"), so は. Nothing about the cat's grammar changed; only its newness did.

Track the age of the information, not its grammatical role

Here is the mental move that separates learners who get this from learners who guess. Do not ask "is this the subject?" — in both sentences above, the cat is the subject. Ask "how old is this information?" A thing that has just entered the conversation is young; a thing already established is old. Young information rides in on が; old information is tracked with は.

Watch the age flip inside a two-sentence stretch. The package（にもつ）is new when it arrives, and known by the next breath:

First mention: 荷物が — the package is news. Second mention, one clause later: 荷物は — now it's old, so we track it with は. This first-が-then-は rhythm is everywhere in real Japanese, from fairy-tale openings to office chat, and it is the clearest proof that age, not role, drives the choice.

The neutral self-introduction is は

When you introduce yourself or state a plain fact about a known topic, you (or the topic) are the given frame and the news is whatever you say about it. That frame takes は.

In both, 私 is the known anchor — you're the one talking, so of course you exist for the listener — and the comment (the name, the preference) is the new part. This is why a plain self-intro is always 私は, never 私が. Reach for 私が here and you accidentally say something much stronger, which is the subject of the next section.

The answer to "who?" is new — so it's が

Flip to a situation where your identity is the missing information. Someone asks which of you is the student; the answer they lack is exactly which person. That newly-supplied identity takes が, and cannot take は.

The student role is already on the table — it's old — but who fills it is the open question, so 私 arrives as new information: 私が. Answer with ×私は学生です and it sounds like you're changing the subject to yourself rather than answering the question. (Why the question word itself is locked to が, and why the answer feels exhaustive — "it's me" — is the job of the exhaustive-focus page, the next stop in this group.)

Fresh, on-the-spot observations take が

A special, very common case of "new information": you notice something happening right now and blurt it out. There's no established topic to frame — the whole event is new — so the default is が.

You are announcing a whole fresh scene. Slip in は here — ×雪は降ってきた — and it sounds like you're contrasting the snow with something else ("the snow, at least, has started…"), which is not what a spontaneous exclamation means. New event, no prior topic → が.

The known referent, revisited with は

The mirror image: when the thing is already shared knowledge, は lets you pick it back up, often with a faint "this one, at least" flavor.

Your wallet is in hand — known — so 財布は. But the keys' whereabouts are the fresh problem you're reporting, so 鍵が. One sentence, both particles, each sorted purely by how new the information is. If you can feel why the wallet is は and the keys are が here, you have the whole page.

The decision procedure, boiled down

Common mistakes

The English "there is a _" is a dead giveaway that the noun is new — a is the new-information article. New existence → が, essentially always.

Keeping が on a second mention is the flip-side error of over-using は: once a thing is established, tracking it with が sounds like you keep re-introducing it from scratch.

Every one of these traces back to the same slip: choosing the particle by grammar (it's the subject!) instead of by information age. Ask the one question — old or new? — and the particle falls out.

Key takeaways

• One question decides most sentences: does the listener already have this information? Old → は, new → が.
• Track the age of the information, not its grammatical role. The subject can be either particle depending on newness.
• A noun's first mention takes が; every mention after that switches to は (the first-が-then-は rhythm).
• Plain self-introductions and known topics take は; fresh observations and answers to "who?/what?" take が.
• This is the info-status blade of は/が. For exhaustive focus ("it's only Tanaka") see the exhaustive-focus page; for behaviour under negation and inside clauses see the scope page; for the full system, the particle page.