Have to is used to express obligation and is similar in meaning to "must" .
But when must is used, it is suggested that the speaker is exercising his
authority:
I have to feed my
dog once a day.
Mother to son: You must make your bed
before you go to the cinema. (The boy has tomake his bed because
his mother says so.)
"have to" needs the auxiliary "do" in questions and negative
sentences:
Mary: I have to study a lot today. And you, John, do you have to
study today? John: No, I don't (have to study today).
have to is used to talk about obligation in the past, the
future, etc. (have can be conjugated in all tenses) while must is used to express obligation in the
present or future:
Yesterday I got up early because I had to wash my car. He
didn't have to study, so he went to the cinema. Robert will have to get up early tomorrow.
"have to" needs the auxiliary "do" in questions and negative
sentences: