Do I Have To?!

When talking about obligations, the usage of have to and must are often confused. So, when should you use have to and when should you use must mean?  

In the affirmative, there is no difference between the two, they mean the same, they mean it’s obligatory. For example:

You have to do your home work = You must do your homework.

(You’re obligated to do your homework.)

WARNING: Like with all modal verbs, you can't use a to infinitive after must,  so you must say: 

I must see a doctor. NOT I must to see a doctor.

But in the negative, there is a difference in meaning. 

Mustn't means it’s prohibited or forbidden, while don't have to means it’s not obligatory, its optional.  For example:

You mustn’t do drive to work. = Driving to work is forbidden.

You don’t have to drive to work. = Driving to work is optional.
Notice that we use an auxiliary verb + not + have to, which we treat as a main verb. You canNOT say: You haven’t to drive to work.

As an alternative to mustn't, you can use can't. For example:

You mustn’t talk in the library. = You can’t talk in the library.

Since we’re talking about can’t, we should talk about can, which we use for permission:

You can talk in the library. = Talking in the library is allowed.


Another important difference between must and have to is that you can only use must in the present, for all other tenses, you must use have to.

For example:

present: You must clean up this mess!

present simple: You have to clean up this mess!

past simple: You had to clean up this mess!

future simple: You will have to clean up this mess!

Another important thing to keep in mind when we ask questions is the position of the modal verbs. As you would expect, can and must go before the subject:

Can’t she come with us? Must she come with us?

But in the case of have to, like when using not, we treat as a main verb, so we need an auxiliary verb: Does she have to come with us?

You canNOT say: Has she to come with us.

To summarize, let’s take a look at the degrees of obligation:

obligatory            must / have to

optional              don’t have to / can

forbidden             mustn’t / can’t