One of the most common mistakes we hear from French speakers is misusing Must and Have to. It is easy to teach and teachers like it because it’s easy to test. (This may seem positive but we think it’s negative as we think you should teach things which will be useful in communication, not things which are easy to grade). Students are often told, and have learned by heart, that Have to implies an external obligation and must implies that there is no external obligation.
It is very easy to test with yes or no/true or false answers.eg You (____________) get a passport to travel abroad. (must/have to)
However, this distinction is not very useful. If you watch a series or movie with English subtitles you will see that often the dialogue uses one form and the subtitles a different one. Students can get annoyed/surprised/upset/irritated by this.
But the real problem is the negative. There is no real difference between “You have to move” and “You must move”. There is a world of difference between “You mustn’t move” and you don’t have to move”.
Don’t have to gives you the choice, it means there is no need to do it but if you wish to do something, you may.
Examples
You mustn’t drink and drive.
You don’t have to drink water during the exam but it’s a good idea.
If you like this, have a look at https://stuartwiffin.substack.com/p/grammar-2-used-to-gerund-infinitive