情态动词 must/have to
语法
情态动词+动词原形情态动词没有人称变化must时态有限过去和现在，have to有现在、过去、将来、完成表否定疑问，直接加not 提前一个句子只能且只有一个情态动词
解释
情态动词表态度情绪must是必须，have to是不得不must和have to 基本可以通用
对比一下
1）must 表示推测“肯定，一定”
She must be 35 years old. 对比:She is 35 years old.
You must be the new teacher.
He looks pale. He must be ill.
2）must 表示不可避免的义务“必须”
I must do it now.（I主语，现在时）
She must do it now.（she主语，现在时）
You must do it tomorrow.（you主语，将来时）
3）have to 表示不可避免的义务“必须”
I have to do it now. （I主语，现在时）
She has to do it now.（she主语，现在时）
You will have to do it tomorrow.（you主语，将来时）
They had to do it yesterday.（They主语，过去时）
4）must推测现在
You must be the new teacher.
They must be waiting for you.
5）must 表示推测“肯定，一定” 推测过去
It must have rained last night.
She must have cried just now.
