は vs が: The Explanation That Finally Sticks

は marks what the sentence is about. が marks which one. Nearly every confusing case follows from those two jobs, and almost no textbook says it that plainly.

Topic versus identification

私は学生です — "as for me, student." You are setting the topic and then saying something about it.

私が学生です — "I am the student." Someone asked which person it is, and you answered.

Same words, different question being answered. が selects out of a set; は sets the frame.

Three places where only one works

• New information takes が. The first time something enters the conversation — 猫がいます, "there's a cat" — it gets が. Once it exists in the conversation, it becomes the topic: 猫は白いです.
• Question words take が, never は. 誰が来ましたか. You cannot make an unknown the topic, because the topic is the thing you both already have in view.
• Contrast takes は. コーヒーは好きですが、お茶は… — the two は's are doing the contrasting.

Why "が is the subject marker" misleads you

Because は often sits where an English subject sits, learners read は as "the subject particle with a hat on". It is not a subject marker at all — it can replace を just as happily: その本は読みました, "that book, I read it." Once you stop mapping these onto English grammar terms, the pattern gets much simpler.

How to actually learn it

Not by rule count. Read forty sentences where the choice matters and, for each one, ask: is this telling me what we are talking about, or which one it is? After a few dozen, the choice starts feeling wrong before you can explain why — which is the point at which you have learnt it.