how many days do you think they will need in shanghai?
问题描述：
how many days do you think they will need in shanghai?
how many days do you think will they need in shanghai?
哪句是对的?为什么?
答
how many days do you think they will need in shanghai?（这句对.）
how many days do you think will they need in shanghai?
原因：
do you think/believe/suppose常在疑问句用充当插入语.因为一个疑问句只有一个是疑问结构（也就是do you think,）而不能再使用第二个疑问结构,所以,就用they will 了.
又如：
What do you think they are doing?
如果去掉do you think,改成：
What are they doing?