I think she must be over forty.有语法错误吗

I think she must be over forty.有语法错误吗 over must forty be she连词成句 I think he must be a fool.这句话有语法错误吗 I think you must be a pretty girl ,don't you?有语法错误吗? I think she must be crazy! 可以说“Sue is more serious,she must be a mature girl.这句话有语法错误吗? 2. I think she must be our new teacher, _______? Trouble will be over,I hope you are happy!这句话有语法错误吗? .—I think the man over there must be Bob. -- It_______ be hi "She must be at least forty ."为什么要在must前加be? that must would be nice有语法错误吗, What must she do 有没有语法错误