I recently saw someone saying “teachers don’t have to make mistakes” while he meant “teachers must not make mistakes.” Although I don’t agree with this statement, I’d like to clarify the difference between must not and don’t have to.
Must is one of the modals in the English language. The word “modal” comes from the word “mood,” especially moods that are grammaticized in languages. Some examples of these moods are certainty (will), requirement (must), obligation (should), suggestion (could or might), opportunity (can or may), etc.
Every modal has its equivalent/alternative. For example, instead of I will, I can say I’m going to, so going to is an equivalent of will. We can say the same about must and have to. For example, I must work and I have to work have the same meaning. However, the confusion comes when must and have to are used in their negative form – must not and don’t have to. Then their meaning changes. I must not work means that I am forbidden from working. I don’t have to work, however, means that I have a choice. That is, I am not obligated to work, but I could if I wanted to.
must vs. have to
Let’s look at some more examples:
A: I must go to the store.
B: I have to go to the store.
Let’s see these two sentences in a context:
Anna: Hey, do you want to grab some coffee?
Bob: I’d love to, but I can’t. I must go to the store. I have some friends coming over tonight and I need to buy groceries.
Anna: But do you really have to go right now? Don’t you have a half an hour?
Bob: Sorry, Anna, I really must go right now because I’ll have to finish a work project later and I’m afraid I won’t be able to be done on time if I go out with you now. Maybe some other time.
In the conversation above must and have to are used interchangeably. In other words, the modal must and its equivalent have to have the same meaning and the speaker can use either one to convey the same message.
must not vs. don’t have to
Now let’s look at must not and don’t have to:
A: I must not work.
B: I don’t have to work.
Let’s look at a dialogue involving don’t have to:
Bob: Hey Anna, do you want me to bring you the documents we worked on yesterday?
Anna: Oh, no worries, you don’t have to do that. I don’t want you to drive back and forth to the office.
Bob: I’m actually in the office right now, so I can take them with me and drop by later.
Anna: Ah, ok, then that would be great.
In the dialogue above Anna tells Bob that he doesn’t have to bring her the documents. But she doesn’t mean that Bob must not do it, which would mean that he is forbidden from doing it. Anna means that he is not obligated to do it because she doesn’t want to cause trouble. But if Bob wants to do it, then she’d appreciate it.
Let’s look at a similar situation using must not:
Bob: Hey Anna, do you want me to bring you the documents we worked on yesterday?
Anna: No Bob, according to the company rules, you must not take any documents out of the building. Remember?
Bob: Oh yeah, thanks for reminding me. I’m glad I asked you. I would have been in trouble otherwise.
In the situation above, Bob is forbidden from taking any documents outside of his company building. Another way to say must not is not allowed to, though this is a slightly weaker version.
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What do you think are the most confusing modals in English? What is the hardest aspect of the English grammar for you to understand and remember?
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