?Key structure
在其他时态时，must的有些含义可以用have to或havegot to。这三种形式一般可以互换，但用于第一人称时，have to(have got to)强调客观的要求或外在的原因，must 则强调主观的要求或表示说话的人认为自己有权做某事。
have to和have got to往往可以互换，但与always，sometimes 等频度副词连用时，用have to往往比用havt got to好。have got to比have to更口语化。
?????????Key structure
在过去时的句子中可以有任意时态
, 要用have to 来表示“必须”，have toShe will have to… to…
；She had to… ；She has/I have had have toto；didn't have to
作为实义动词, 否定式为don't have to；will not have 在表达“难道你不能不（做you…?”，而不用“Do you have to…?……）”时，一般用“”
Must Must you leave now? 难道你非得现在就走吗？I’m afraid so. I have to study for an exam.
mustdone可以表示推测，表示对过去的推测must do表示对现在的推测；must have He must be mad.
在说成I think…/he thinks…“I think he must be a fool.的从句中，一定要用事实说话”)
. (不能???Key structure
的否定式为can’t be（不可能）：’s using the phone, it must be John.’t be John. He promised me he wouldn’t must beSomeoneIt canuse it today.
Special Difficulties
As的用法
①作为介词，可以表示“作为”、“以……身份”等In this film, he appeared as a policeman.
As a mechanic, he can’t always keep himself clean.
②作为连词，可以表示“因为”、“正当……时候”、“以……方或“如同……那样”等
You must take care of the bady tomorrow as I am going to London（因为，由于）
As we were talking about him, he knocked at the door.（正当……时候）
Do as you are told. 叫你怎么做就怎么做(按你被告知的那样去做)（以……方式）
As I learned, … （如同……那样）③“担任……工作”I work as an engineer.