2. I think she must be our new teacher, _______?

1 首先,must在这里是表猜测的,解释为“肯定,一定”,不解释为“必须”.
2 再看看 she must be our new teacher的原始句子：she must be our new teacher== I am sure she is our new teacher.
3 这样一来 I am sure 是插入语成分,你可以忽略掉,she is our new teacher的反意疑问句结构是：she is our new teacher,isn't she?
4 再回到你的句子 I think she must be our new teacher,ISN'T SHE --------- i think是插入语成分,你可以忽略掉.这些刚才讲的就构成了反意疑问句结构.