【英语语法】情态动词 must , mustn't , 和don't need to do 的用法

• must 必须做某事 ,
• musn't  不必须做某事，
• don't need to (过去式）不必须做某事      

例句：

You don't need to go. You can stay here if you want.

It's fantastic film. You must see it.

It's lisa birthday next week . I mustn't  forget to buy her a present.