谁

Use 谁 to ask 'who'. What is convenient about question words in Chinese is that you simply replace 谁 into the position of the person you want to ask about in a declarative sentence — no inversion is needed as in Vietnamese or English. For example, if the answer is 他是老师 (He is a teacher), the question would be 他是谁? (Who is he?).

谁 has two readings: shuí (formal) and shéi (colloquial).