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 is not merely a more emphatic version of 私(わたし)は. It answers “who?” by selecting one person.

When が introduces something

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.

Common mistake

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.

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