Spotting It, Asking About It が・は

が here marks something just noticed, a bus pulling up, flowers suddenly in view. This is the same logic behind Old News, New News in Korean: が announces something new, while は would sound strange here, as if the bus or the blossoms were already the agreed topic of conversation.

は sets the topic first, the cafeteria, the bathroom, then asks or says something about it. These questions aren’t spotting something new, they’re asking for a detail about a thing already under discussion, which is exactly the job は does.

Inside a clause that modifies a noun, the subject of that inner clause always takes が, never は, even when the whole sentence’s real topic is marked with は elsewhere. これは [妹が作った] ケーキです nests a full が-marked clause, “my sister made it,” inside the bigger は-marked sentence, “this is a cake.” This is the same が you’ll meet again in Relative Clauses.