do you think 是用在非凡疑问句中的插入语，用来征询见解或表达看法。因为句中插入语已经是疑问式，主谓倒装了，所以you are的位置不能再对调了。插入语一般是对句子加一些附加的解释，在句子中作独立成分。常用来作插入语的结构有I hope, I think, I'm afraid, you know等。插入语可以位于句尾或句中，有时也可出现在疑问句中。
2．must have been doing sth.,一定/准是在做某事。
这种结构用来表示对过去正进行的事情的推测。下文中的can't have been是 can't have been driving的省略式，在意思上与must have been driving正相反。
3．That's why I didn't see the sign.所以我才没看见那牌子。
why I didn't see the sign作is的表语从句，以why引导的从句用来解释事情的原因。
4．…you'd better take my advice!……你最好还是服从我的劝告吧！
take one's advice是“服从劝告”的意思。
关键句型
表示猜测和推断的情态助动词must和can't（2）
must与can't不仅能表达对现在和将来的猜测和推断，而且能表达对过去的猜测和推断——这时可用 must have been表示肯定的推断，用can't have been（而不是mustn't have been）来表示否定的推断，如后面跟进行时则表示对过去正在进行的动作的猜测。请看例句：
You must have been driving at seventy miles an hour.
你当时肯定正以每小时70英里的速度行驶。
I didn't see it. I must have been dreaming.
我没看见。我当时一定是在胡思乱想吧。
She can't have been 29.She must have been 36.
她那时肯定不会是29岁，她一定36岁了。
He can't have been reading. He must have been sleeping.