So we have looked at the modal verb “must” and the semi-modal verb “have to” and have seen that they serve very similar roles. They both talk about an obligation – something that is needed to be done. Naturally the question arises: are they the same thing?
What are the differences?
While they do indeed both show obligation, they are not used the same way by native English speakers. There are two main differences: formality and urgency.
The first difference is with something called “register” meaning the formality and the choice of words to fit the situation. The way you speak with close friends at your or their home is different from how you would address a complete stranger in public (or at least, it is for most people).
When we compare “must” and “have to”, “have to” has a lower register (i.e. less formal) than “must”. Because of this, it is more normal for native English speakers to use “have to” when speaking. For example: “I have to use the toilet” rather than “I must use the toilet”. The first example is less formal and is more normal/comfortable for native speakers to use in most contexts.
If “have to” has a lower register, “must” has a higher register. Consequently, it is used more often in writing (with the exception of writing that reflects the spoken word, such as SMS or social media posts).
In the end it means that you are more likely to hear “have to” in a conversation and see “must” in a letter rather than the other way round.
While “have to” is more common when speaking, it has a lower sense of urgency than “must”. It is therefore possible to use “must” when speaking to show that something is even more urgent than just using “have to” in the same sentence. A good example of this is in Hollywood films. Let me set the atmosphere: aliens are attacking the White House. They will arrive within the next 10 minutes and the President needs to evacuate onto Air Force One. For some reason the president is taking too long to evacuate in these types of films (giving further orders, checking on friends and family, etc.) and the Secret Service is trying to hurry him along. They might say the following sentences:
We have to go, Mr. President
We need to go, Mr. President
We must go, Mr. President! – the most urgent
The first one, with “have to”, shows an obligation but there is no urgency. When they found out originally about the alien invasion, they might be less worried, especially if the aliens start on the US West Coast. They have time to respond. As the invasion progresses, the agents would switch to “We need to go”.
For this sentence there is often a stress when saying ‘need’ in this sentence. In writing this looks like this: “We need to go, Mr. President.” When speaking you accentuate the /n/ sound and elongate the /iː/ sound (the double -e in this case). Note that the verb “need” is not a modal verb because it uses the full infinitive of the verb “to go” afterwards.
When the alien spaceships are in sight of Washington DC, the Secret Service would switch to “We must go, Mr. President” and most likely would physically pick him up. This is because this phrase shows the most urgency and it needs to happen now.
Want to learn more about modal verbs? You can find a guide to all the semi-modal verbs here: introduction to semi-modal verbs. You can also learn more about other English language information on my Teaching English page.