Contents introduction chapter I grammatical meaning, grammatical form

Infinitive; to have to leave
Inf form; having to leave
Present; they must leave
Future; they must leave tomorrow
Perfect; they have had to leave
Past; they had to leave
Past perfect; they had had to leave Future perfect; they will; have had to leave Conditional; they would have had to leave
Chapter II Grammatical category
2.1. The usage of modal verbs in expressing obligation and necessity.
Each of the modal verbs expresses different levels, such as obligation, advice, necessity, ability, permission and others. Some modal verbs are used to show speaker's obligation which he or she must, need, have to, be obliged to, should, ought to, be to do something generally or at that moment. When we speak about our obligation, we use these modal verbs. And these modal verbs help human to be beautiful their speech and abbreviate their sentences.
When you want to say that someone has an obligation to do something, or that it is necessary for them to do it, you use 'must' or 'have to'.
You must come to the meeting tomorrow.
The plants must have plenty of sunshine.
I enjoy parties, unless I have to make a speech.
He has to travel to find work. There is sometimes a difference between 'must' and 'have to'. When you are stating your own opinion that something is an obligation or a necessity, you normally use 'must'.
/ must be very careful not to upset him.
We must eat before we go.
He must stop working so hard. When you are giving information about what someone else considers to be an obligation or a necessity, you normally use 'have to'.
They have to pay the bill by Thursday.
She has to go now. Note that you normally use 'have to' for things that happen repeatedly, especially with adverbs of frequency such as 'often', 'always', and 'regularly'⁷.
/ always have to do the shopping.
You often have to wait a long time for a bus.
⁷ Merriam- Webster's Concise Dictionary of English Usage. Merriam- Webster. 2002. p. 98.
You use 'must not' or 'mustn't' to say that it is important that something is not done or does not happen.
You must not talk about politics. They mustn 'tfind out that I came here. Note that 'must not' does not mean the same as 'not have to'. If you 'must not' do something, it is important that you do not do it.
If you 'do not have to' do something, it is not necessary for you to do it, but you can do it if you want.
You only use 'must' for obligation and necessity in the present and the future. When you want to talk about obligation and necessity in the past, you use 'had to' rather than 'must'.
She had to catch the six o'clock train. I had to wear a suit. You use 'do', 'does', or 'did' when you want to make a question using 'have to' and 'not have to'.
How often do you have to buy petrol for the car? Does he have to take so long to get ready? What did you have to do? Don't you have to be there at one o'clock? You do not normally form questions like these by putting a form of'have' before the subject. For example, you do not normally say 'How often have you to buy petrol?'
In informal English, you can use ' have got to' instead of 'have to'. You've just got to make sure you tell him. She's got to see the doctor. Have you got to go so soon? You normally use 'had to', not 'had got to', for the past. He had to know. I had to lend him some money.
You can only use 'have to', not 'must', if you are using another modal, or if you want to use an '-ing' form⁸, a past participle, or a 'to'-infinitive. They may have to be paid by cheque. She grumbled a lot about having to stay abroad. I would have had to go through London. He doesn't like to have to do the same job every day. You can use 'need to' to talk about the necessity of doing something. You might need to see a doctor. A number of questions need to be asked. You use 'don't have to' when there is no obligation or necessity to do something.
Many women don't have to work. You don't have to learn any new typing skills. You can also use 'don't need to', 'haven't got to', or 'needn't' to say that there is no obligation or necessity to do something. You don't need to buy anything. I haven't got to go to work today. I can pick John up. You needn 't bother. You also use 'needn't' when you are giving someone permission not to do something.
You needn't say anything if you don't want to. You needn't stay any longer tonight. You use 'need not have' or 'needn't have' and a past participle to say that someone did something which was not necessary. You are often implying that the person did not know at the time that their action was not necessary. / needn't have waited until the game began. Nell needn't have worked. They needn't have worried about Reagan.
⁸ Fleischman, Suzanne, The Future in Thought and Action, Cambridge Univ. Press, 1982, p. 102.
You use 'didn't need to' to say that something was not necessary, and that it was known at the time that the action was not necessary. You do not know if the action was done, unless you are given more information. They didn 't need to talk about it.
I didn't need to worry.
You also use ' didn't have to' to say that it was not necessary to do something.
He didn't have to speak.
Bill and I didn't have to pay.
You cannot use 'must' to refer to the past, so when you want to say that it was important that something did not happen or was not done, you use other expressions.
You can say 'It was important not to', or use phrases like 'had to make sure' or 'had to make certain' in a negative sentence.
It was important not to take the game too seriously.
It was necessary that no one was aware of being watched.
You had to make sure that you didn't spend too much.
We had to do our best to make certain that it wasn 't out of date.