oxford_guide_to_english_grammar

12 MODAL VERBS PAGE l16
When we talk about necessity in the present or the near future, we can use either
must or have (got) to. But there is a difference in meaning. We normally use must
when the speaker feels the necessity and have to when the necessity is outside the
speaker.
You must buy your ticket before starting your journey.
I have to buy the ticket before I get on the train.
The leaflet uses must because the rule is made by British Rail, and they are the
authority. Phil uses have to because the rule is not his, and the necessity results
from the situation.
You must... is a way of ordering someone to do something. You have to... is a way
of telling them what is necessary in the situation.
You must fill in a form. (I'm telling you.)
You have to fill in a form. (That's the rule.)
I must go on a diet. I'm getting overweight.
I have to go on a diet. The doctor has told me to.
NOTE
a Compare the meaning of must and have to in questions.
Must I write these letters now? (= Do you insist that I write them?)
Do I have to write these letters now? (= Is it necessary for me to write them?)
b We can also use be to for an order by a person in authority. • 76(2)
The doctor says I'm to go on a diet.
But have to is much more common than be to.
c Be obliged to and be required to also express necessity. Both expressions are rather formal.
You are obliged to/are required to sign a declaration.
b We sometimes use must "for things we think are necessary because they are so
enjoyable.
You really must watch this new Canadian soap opera.
We must have lunch together.
c
Must has no past tense, no perfect or continuous form and no infinitive or
ing-form. We use have to instead.
I had to pay £15 for this railcard last week.
We've had to make a few changes.
I'm having to spend a lot of time travelling.
I wasn't expecting to have to look after the children.
It's no fun having to stand the whole journey.
You will have to pay the full standard single fare.
2 Have to and have got to
a
b
Both have to and have got to express the same meaning: necessity which is outside
the speaker.
I have to take an exam in June.
I have got to take/I've got to take an exam in June.
Have to is common in both formal and informal English, but have got to is informal.
We use got only in simple tenses, but have to has all the forms of an ordinary verb.
Father was so ill we were having to sit up with him night after night.
I don't want to have to punish you.
We cannot use got here.