must有过去式吗?can,may都有过去式,must怎么办.比如：she was a hard-working student,because she believed that she ____ work hard.她相信她必须好好学习.这个"必须"怎么填...

must有过去式吗?can,may都有过去式,must怎么办.比如：she was a hard-working student,because she believed that she ____ work hard.她相信她必须好好学习.这个必须怎么填... can may must 过去式是什么 must和may有没有过去式啊? must有过去式吗 有must/could/can/may/maight have been done吗 “can”有过去式吗? may是must的过去式吗? 请问can有过去式吗? do与can、will、would、shall、may、must助动词用法一样吗 有什么区别 The salesgirl a mistake.选项有 can make may make must make must have made may;must;shall;的过去式 can、may、must开头的问句怎么回答?can't和musn't有什么区别? could除了can的过去式还有什么理解 can could may must should的区别? 英语中should,would,must,need,can,may怎么用?否定句呢?这些词有什么区别吗， can be,may be和must be这几个词都有什么用法?在使用上有区别吗? 为什么will,shall.can.may没有过去分词和现在分词而有过去式 在英语的情态动词中,过去式和原型的意思完全一样吗比如说can,could.may,might,will,would.还有他们的否定形式,比如说can,cannot.must,mustnot,should,shouldnot是不是都是对原型的否定,有没有什么不一样的? can't have been和can' have done有什么区别或者must have been和must hane done;may have been和may have done