must 与 have to 都有"必须、应该"的意思.但二者有区别：
1) must 侧重于个人意志和主观上的“必须”,have to 则侧重客观上的必须,可译为“不得不”.must 与 have to 经常可以互换使用.例如：
I must/have to go to school now.现在我必须上学了.
2) must 可用来表示现在和将来的必须,无时态变化；have to 则有更多的时态变化.例如：
He had to give up the plan.他不得不放弃这个计划.
I will have to buy a new pen.我不得不买枝新笔.
3) 变否定句时,must 只要直接在其后加 not,而 have to 既可变成 haven't to,也可变成 don't have to / doesn't have to.
