The modals “must” and “have to” talk about obligation.[1]https://www.englishclub.com/grammar/verbs-modals-have-to-must-not.htm The difference between the two types of obligation is if it internal or external.
Must
In general, you use “must” to talk about personal opinion. This is subjective. That means, the speaker thinks it is necessary.
For example:
- I must study; otherwise, I won’t do well on the test.
(The implication is that the speaker wants to do well on the test. Therefore, they are going to study. It is not a requirement for them to study.)
- You must try this sandwich! It is so tasty!
(The implication is that you really should try the sandwich that I think is very tasty. However, if you don’t want to, you are required to.)
- If I am going to become a professional basketball player, I must practice every day.
(The implication is that it is necessary to practice every day in order to become a professional basketball player. However, neither becoming a professional basketball player nor practicing every day is a requirement.)
Have to
In general, you use “have to” to talk about external rules or opinions. In other words, you use it when rules and other people are telling you to do something. It is not a personal opinion.
For example:
- In England, you have to drive on the left hand size of the road.
(The implication is that it is the law in England to drive on the left hand side of the road.)
- I have to wear a uniform at work.
(The implication is that it a requirement to wear a uniform at work.)
- You have to wait for the stop light to turn green before you can move forward.
(It is the law that you can’t drive through a red light. You must wait for the green light to drive.)
Practice Exercises
References