Study materials
You are in a museum and you see a sign. Two very similar sentences mean opposite things, and they decide whether your phone comes out of your pocket.
You mustn't take photos here.
You don't have to take photos here.
In the positive, must and have to are near twins. In the negative they pull in different directions, and that is the most important thing in this topic.
Must and have to: who is doing the demanding
Both mean obligation, but they look at it from different sides. Must is a demand from yourself or from the speaker. Have to is a rule from outside: a law, a job, the circumstances.
I must stop drinking so much coffee.
We have to wear badges at work.
After must comes the base form straight away, with no to. Have to, on the other hand, behaves like an ordinary verb: it changes with the person and needs do in questions.
She has to leave early.
Do you have to work on Saturday?
Have to lives in every tense, must does not
Here is the practical reason you hear have to more often. Must exists only in the present, while have to moves freely into the past and the future.
We had to wait two hours.
You'll have to ask the manager.
So yesterday's obligation is always had to. There is no such form as musted.
Should: advice, not an order
Should is weaker than either of the other two. It is what is worth doing, not what you will have to do. You are advising, not demanding.
You should rest.
You shouldn't eat so late.
Should I call a doctor?
Should behaves like must: the base form follows it without to, and in a question it simply moves in front of the subject.
The negatives pull apart
Now let us put all three negatives together. They sound alike and mean quite different things.
| Form | What it means |
|---|---|
| mustn't | it is forbidden |
| don't have to | it is not necessary, you may skip it |
| shouldn't | it is better not to |
You mustn't park here.
You don't have to park here.
You shouldn't park here.
For an absent obligation in the past, the form is didn't have to.
I didn't have to pay.
The mistakes everyone makes
The first is putting to after must or should.
You must to go. → You must go.
The second is using don't have to where a ban is meant.
You don't have to smoke here. → You mustn't smoke here.
The third is trying to put must into the past.
I must work late yesterday. → I had to work late yesterday.
All three are caught by two questions. Is this a ban or simply no need? And is it about the past? A ban is mustn't; the past is had to.
Try it right now
Think of one rule from your job or from the building you live in.
Say three sentences about it in English: what is forbidden, what is not required, and what you would advise a newcomer. Three forms, three different levels of pressure.