Must / have to
These have similar meanings. Sometimes they can be used interchangeably [互換的に], but not at other times. “Must” is stronger than “have to”. “Have to” means “need to”. In other words, it is necessary but not required. “Must” implies [ほのめかす] a requirement [要件].
I must go to the dentist.
I have to go to the dentist.
I need to go to the dentist.
I must remember to buy milk.
I have to remember to buy milk.
I need to remember to buy milk.
First you must go to Immigration.
First you have to go to Immigration.
First you need to go to Immigration.
Must I read everything?
Do I have to read everything?
Do I need to read everything?
You must not speak so loudly. そんなに大声で話してはいけません。
You don’t have to speak so loudly. 大声で話す必要はありません。
You don’t need to speak so loudly. 大声で話す必要はありません。
I must not forget to renew my driver’s license.
I have to / need to remember to renew my driver’s license.
[We cannot say, “I don’t have to forget…” because that would have the opposite meaning of “it would be OK to forget”.]
The children must not go to the park alone.
[This is a strong command or requirement, so have to / need to cannot be used because to use them would change the meaning just like in the previous example.]
Must can also be used as a noun meaning “requirement”.
Example: “That is a must.”