先生

How it works

This is a soft, casual question. 先生(せんせい) (teacher) is followed by な (is (before の)), the form the plain copula だ (is) takes before the explanatory の (you mean?). That final の (you mean?), said with a rising tone, asks for confirmation in a gentle, curious way — “you’re a teacher?” rather than a blunt fact-check. No polite です (is), so it sounds relaxed and personal.