情态动词must用法
(1) 表示“必须”
此时可用于肯定句、否定句或疑问句,用于否定句时,mustn’t的意思是“一定不要”“不能”,而不表示“不必”.
如：Water must be pure if it is to be drunk.水如供饮用必须净化.
Must I finish the work tonight?我必须今晚完成这项工作吗?
Adverbs mustn’t be put between the verb and the object.副词不能放在动词和宾语之间.
must主要表示现在或将来,但在间接引语中也可表过去(=had to).
如：She asked her boss if she must (had to) work overtime.她请示上司她是否要加班.
对于must开头的一般疑问句,若作否定回答,可以用needn’t,不用 mustn’t.
如：“Must I clean all the rooms?” “No,you needn’t.” “这些房间我都得清扫吗?”“不必.”
(2) 表示推测 意为“准是”“一定是”,通常只用于肯定句.若用于否定句或疑问句,则用 can 代之.
如：That can’t be the only way.There must be other ways of solving the problem.那不是解决问题的唯一办法,肯定有另外的办法.
Jane’s light is on.She must be at home.She can’t be out.简的房间里开着灯.她一定在家,不会出去的.
若要谈论过去的情况,必须用“must+have+过去分词”.
如：You must have mistaken my intention.你一定是误会了我的意图.
She must have been very young when she got married.她结婚时一定很年轻.
He must have come this way; here are his footprints.他一定是走这条路来的,这里还有他的脚印呢.
(3) 表示“偏偏” 有时must可表示某事发生得不早不迟,就在某个当紧的时候,多指某些不受欢迎的事情的发生.
如：The car must break down just we were starting our holidays.我们刚刚开始休假,汽车偏偏坏了.
Must you worry her with questions,just when she is busy cooking dinner?她正忙着做饭你干吗偏偏问她问题?
Just when I was busy,the neighbor must come and chatter.正当我忙碌的时候,邻居偏偏过来聊天.
有时还可指主观上的偏执或固执,常可译为“偏”“偏要”.
如：After I gave her my advice,she must go and do the opposite.在我给她出主意之后她偏要反着干.
Must you make so much noise?你就非得弄出这么大的声音吗?