は or が? Start with what the listener needs to identify

は usually sets a topic, while が marks the subject and often identifies the person or thing that answers an open question. That is why both 私(わたし)は行(い)きます and 私(わたし)が行(い)きます can be grammatical but do different conversational work. The first tells you something about me. The second identifies me as the one who will go.

Think about the missing information

A useful way to choose is to ask what the listener still needs. If the person, place, or thing is already the topic, は lets you continue talking about it. If the identity itself is the new answer, が points to it.

Imagine a conversation as a labeled folder. は writes the folder label: “as for this.” が places a spotlight on the item that satisfies the sentence.

Here, 私(わたし) is the topic. The sentence contributes the information 学生(がくせい)です.

The question leaves the identity open. 田中(たなか)さんが fills that gap. In natural English, extra stress on “Tanaka” often carries the same effect.

The contrast in a real exchange

This can contrast your plan with other people’s plans, or simply continue a conversation whose topic is you.

Here, 私(わたし)が answers “who?” by selecting one person out of the possible candidates.

When が presents something new

が also commonly introduces something newly noticed or newly relevant.

The cat has not been established as the topic. The sentence presents it. Once the cat is part of the conversation, you can shift to 猫(ねこ)は and say more about it.

Work that through with a scene. You are walking home from the station and you pass a small park. Nothing has been said about the park, and nothing has been said about animals. Then you notice something on a bench.

Your listener has no cat in mind, so there is no topic to carry forward. The sentence hands them the cat and the place at the same time. Marking the cat with は would tell them you are already talking about a particular cat, and they would have to work out which one you mean.

After that sentence, the cat is shared ground between you, and the next sentence can treat it as the topic.

Practice those two as a pair rather than as two separate facts. The first sentence presents, the second comments. The switch to は does not correct the first sentence. It records that the cat has moved from new information to something you both hold.

Events enter a conversation the same way. Something you have noticed this second comes in with が, because you are announcing it rather than adding to a subject your listener is already carrying.

Said on a platform to the person next to you, that sentence puts the train into the conversation. From there, the sentences that follow can treat the train as the topic and say what time it left or how crowded it was.

When は holds two things apart

Continuing a topic is one job. は has a second one: it can hold a pair of items up against each other.

Start from a question you can already answer. A relative is counting how many people to expect at a family dinner.

That answer selects one person out of everyone who might come, the same work the earlier answer about going does. Now keep the facts exactly as they are and mark two people with は.

Neither sentence picks a person out of a lineup. Each one takes someone the listener can already identify and reports something about them, and the matching は marking is what makes the two reports land as a pair: one yes, one no, set side by side.

That pairing is also why a single は sentence can carry a contrast with nothing negative in it. If your listener is holding several people or several options in mind and you mark only one of them with は, the sentence describes that one and leaves the rest standing in the background. That is the reading the earlier sentence about going picks up when other people’s plans are in the air.

A topic can have a subject of its own

The two particles are not competing for one slot. A single sentence can set a topic with は and mark the subject the predicate describes with が.

は sets the frame: we are talking about me. Inside that frame, the language is the thing the understanding applies to, and it takes が. Both particles are doing their ordinary job in one sentence.

The same shape works when the comment is a description instead of an action.

The shop is the frame. The coffee is the thing the description is true of. Marking the shop with が would push the sentence toward answering which shop, and that is a different question from the one you set out to answer.

The が slot in this pattern is the same open slot from the start of the lesson, which means a question word can sit in it.

The topic is settled before the question is asked: we are talking about this shop. What the speaker wants is the item that fills the が slot, and an answer fills it with a specific thing instead of changing the subject.

Rules such as “は means is” or “が means emphasis” will fail quickly. Neither particle has one fixed English replacement. Listen for the conversational job: are you setting a topic, or identifying the subject that makes the statement true?

A practical decision

Use this sequence when you hesitate. First, look at the previous sentence or question. If the listener already knows what you are talking about and you are adding information about it, は is a strong candidate. If the listener needs to know who or what, が is a strong candidate. Then check whether the sentence contains a larger topic with its own subject.

Two more checks now that the other jobs are on the table. If the thing you are about to mention has not come up at all, you are presenting it, and が does that work. If you are holding two items apart on purpose, marking both of them with は is what sets them side by side.

You will still meet patterns that need their own explanation, including は inside a subordinate clause. This starting point is useful because it follows the information in the conversation instead of pretending the particles are interchangeable labels.