情态动词 must/have to

语法

  1. 情态动词+动词原形
  2. 情态动词没有人称变化
  3. must时态有限过去和现在，have to有现在、过去、将来、完成
  4. 表否定疑问，直接加not 提前
  5. 一个句子只能且只有一个情态动词

解释

  1. 情态动词表态度情绪
  2. must是必须，have to是不得不
  3. must和have to 基本可以通用

对比一下

17情态动词 must/have to_i

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.