The modal verbs Must and must not are an interesting modal pair. While it looks like they should have opposite meanings (since one is positive and one is negative), they actually have the same meaning! This can be difficult for beginners to understand.
Let us first look at the uses of must.
Must for Obligations
The main use of this modal pair is to show an obligation or a necessity to do something. For example:
- I must pick up my sister from school.
- I must go home after my dentist appointment.
In both cases something has to be done. In the first case the speaker has to pick up his sister. There is no way that the person can get out of this situation. In the second case the speaker is at a dentist appointment and needs to go home afterwards. Again there is no way that the person can get out of this situation.
We can also make assumptions using must. With these assumptions we are not completely sure about what is being said but we are fairly certain. For example, if we see someone in a dentist’s office, we could say:
- She must be here for a dentist appointment.
Because of the information that we have at the time, we can make a confident assumption. This can be very useful when you are certain about information based on what you know or observe without being completely sure.
Confident assumption in the past
When you use the verbs must and mustn’t with the present perfect, the meaning of the sentence becomes a confident assumption in the past. In other words, the speaker is not completely sure about what happened in the past but they can make an educated guess. Let’s use an example about someone who was at a party:
- She must have left the party early.
- Susan must not have walked home from the party. She got home very quickly.
In the first sentence, we assume based on evidence that she left the party early. Maybe we are now at the party and we can’t find her. If we can’t find her, we can make the confident assumption that she has left. Imagine now that she has sent us a message saying that she is home but we saw her 10 minutes ago and she lives across the city. For the second example, we can confidently assume that something did not happen. She did not walk home because, if she did, she would still be walking home.
Why/How are must and mustn’t the same?
If must not has the word ‘not’, why does it mean the same thing as must? It’s because the word not is not a negation for the modal verb must but for the main verb (the one with meaning). This means that it is still an obligation, an obligation not to do something. For example:
- You must do your homework. – The obligation is to do homework.
- You must not do your homework. – The obligation is to not do your homework.
These both show that there is a necessity or an obligation. In the first sentence, something has to be done. In the second sentence, something has to not be done. The function of the modal verb remains the same whether it is must or must not. It is the thing that has to be done that changes from the positive to the negative when not is added.
Want to learn more about modal verbs? You can find a guide to all the modal verbs here: introduction to modal verbs. You can also learn more about other English language information on my English Language Matierals page.