は or が

は marks the topic, meaning what the sentence is about. が marks the subject when the subject itself is the new or important information.

• Use が after a question word that is the subject (誰が, 何が, どれが), and in the answer to such a question.
• Use が for something you have just noticed or that has just happened (バスが来ました), for existence with あります/います, and with likes and body feelings: 猫が好き (I like cats), のどが痛い (my throat hurts), おなかがすいた (I'm hungry).
• Use は for the topic of a question whose question word is in the predicate (トイレはどこですか), for definitions and general facts, and for contrast (肉は食べますが、魚は食べません).
• Inside a clause that describes a noun, or an if/when clause, the subject never takes は. It takes が, or の in a noun-describing clause: 母が作ったケーキ (a cake my mother made).