4 Mistakes English Learners make with ‘must’ and obligation

One of the main problems that English learners have is using modal verbs correctly, and ‘must’ is no exception. Here are the main mistakes that learners make with must and why you should avoid them!

Mistake #1. Saying ‘must to’

First of all, lets start with the easy ones, we NEVER say ‘must to’

With all modal verbs we never use ‘to’ after: Sentences like ‘I must to study’  look very strange, this should be ‘I must study’.  

This mistake will always be understood, but sounds quite strange.

You can, however, say ‘I have to study’.  Have to and must  mean the same thing.

Mistake #2. Saying ‘will must’

If you want to talk about things which you must do in the future, you can NEVER say ‘will must’

There are 2 possibilities, you can either just use must/have to: ‘I must/have to do some work tonight’, this works best when you MAKE THE DECISION NOW about something you will do in the future.

You can also use ‘will have to’: ‘We will have to show our passports when we arrive at the airport’ which works best when you talk about something which WILL BE DECIDED IN THE FUTURE.

The difference between these isn’t so important, if you are unsure it is best to use ‘must’ to talk about the future.

Mistake #3. Using ‘have to’ when you should use ‘must’

As I just wrote above, it is possible to use must or have to to talk about necessary things in the future.  But there is one very small difference between must and have to, which can really change the meaning.  We only use ‘have to’ WHEN AN AUTHORITY DECIDES that something is necessary, for example:

‘I have to go to work tomorrow’  (your boss decides it is necessary)

‘I have to fill in my tax forms’ (the government decides it is necessary)

for sentences where an authority decides, you also have the option to use must: ‘I must go to work tomorrow’ / ‘I must fill in my tax forms’

So whenever an authority decides that something is necessary, you can choose between must or have to.


IF YOU PERSONALLY decide that something is necessary, then it is so much better to use must, for example:

‘I must go to the gym’ (I decide it is necessary, no-one is telling me to go there)

‘I must lose some weight’ (I decide I must lose weight, there is no law to say I must do it!)

Often the difference between these doesn’t really matter, but when you want to talk about the fact that you want to call your mother because you haven’t spoken to her for a week, it can make a big difference!

If you say ‘I have to speak to my mother’ It sounds like your mother is telling you that you have to call her, which actually sounds like you don’t want to call her! Here it is really necessary to say ‘I must call my mother’

Something like ‘I have to visit my friend in hospital’ (sounds like your friend is telling you to visit him, which sounds like you don’t want to visit him).  Here it is really necessary to say ‘I must visit my friend in hospital’

So, in summary, here is a chart to show you when you can use must and when you can use have to:

                      Authority decides         You decide

must                  GOOD                              GOOD

have to              GOOD                               BAD 

Why not test your new knowledge with a ‘must’ or ‘have to’ quiz!

Mistake #4. Using ‘must have’ instead of had to

When talking about things which were necessary in the past we use ‘had to’

‘When we arrived at the airport we had to show our passports’

If you use ‘must have’ here instead of ‘had to’ it sounds very strange and it completely changes the meaning.  Must have is used to talk about possibility and not obligation.  It means that there is a strong possibility that something happened.  So if you use the above sentence with must have it makes no sense because the meaning is completely different:

‘When we arrived at the airport we must have shown our passports’  means ‘when we arrived at the airport there is a strong possibility / I am almost certain we showed our passports.’  It sounds like you are trying to remember whether you showed your passports or not when you arrived at the airport.

This error can sound even more strange in other examples, take this correct sentence with ‘had to’:

‘I’m sorry I’m late I had to visit the dentist’

If you say this with ‘must have’: ‘I’m sorry I’m late I must have visited the dentist’, it sounds like you are trying to remember why you are late, and you are almost certain it is because you visited the dentist.  It actually sounds like you can’t remember something that just happened.

In a similar way with this sentence:

 ‘I missed the bus so I had to walk home’.

If you use ‘must have’ here: ‘I missed the bus so I must have walked’ it sounds like you are trying to remember how you got home, quite worrying!

So it is very important to make sure you get this last mistake right, because it can make some very strange sentences if you get it wrong!  Check out some new phrases with must that you may not have seen before with my guide to business phrases with modal verbs!

If you would like to have any of these words explained further, or if you would like to see some more examples, feel free to email me at [email protected], or message me on Skype at live:fluencyspace! Also if you have any other questions about English I’m happy to answer your emails or I will write a post about it, keep the emails coming! Click here for more advanced Business English grammar!


 David Cox

 Fluency Space

 Make the world your fluency space. Business English for career and life success

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