主语是their teacher
他们的老师是谁？
类似于：
Who is he?
Who is the boy?
Who作主语时，一般先译who
如：
Who will come?谁会来？
名词作表语 本来就可以 和主语调换 句意不变。
所以本句主语可以是who 也可以是their teacher
本句表语可以是who 也可以是their teacher
根据who所代替的句子成分,和回答而定
如mr wang is their teacher.
their teacher is mr wang.
再如
the woman is their teacher.
对主语提问who is their teacher?主语是who表语是their teacher
对表语提问who is the woman?主语是the woman表语是who
再如
could you tell me who is their teacher?
could you tell me who their teacher is?
前句who是宾从中的主语；后句who是宾从中的表语。两句都对。
who是主语
他们的老师是谁
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