〜は vs 〜が
は marks the topic of the sentence that is already known to the listener or introduces a contrast, placing communicative focus on the information that follows. が marks the grammatical subject or identifies new information, placing focus directly on the noun preceding が. Swapping them changes whether you are commenting on an established topic or identifying who or what specifically performs the action.
〜は topic: as for …
田中さんは親切な先生です。
猫は好きですが、犬は好きではありません。
〜が subject: the one who / that which
田中さんが新しい先生です。
ほら、あそこに可愛い猫がいますよ。