「が」marks the subject with focus or emphasis — often introducing new information. ① Focus on subject: Dare ga kimashita ka? → Tanaka-san ga kimashita. (Who came? Tanaka came.) ② Likes / ability: Neko ga suki desu. / Nihongo ga wakarimasu. ③ Existence: Tsukue no ue ni hon ga arimasu. 💡 は vs が: は＝known topic (what we already know), が＝new info / subject you want to emphasize