1. Obligation
Unit 4

2. Must and have to
We use have to / must + infinitive to talk about obligation, things that are necessary to do.
Must and have to are often quite similar. They are both followed by the infinitive.
I must go now. / I have to go now.
WARNING: They are not exactly the same. We often use must for more personal opinions about what it is necessary to do, and have to for external obligations, what somebody in authority has said it is necessary to do.
I must remember to get a present for Daisy.    (my opinion)    You have to look after their hair regularly.     (dog experts say so) Do you have to wear a tie for school?    (asking about school rules)

3. Which verb do people use more?
Have to is more frequent in conversation; must is used more in formal writing, for example in written notices.
Passengers must fasten their seat-belts .
Do they change in form for I, you, he, she, etc.?
Have changes in the third person singular (he/she/it has); but must doesn’t change. It’s a modal verb and modals don’t change.
I must I have to
You must you have to
She, he, it must She, he, it has to
We must we have to
They must they have to

4. There’s something very important about must and have to
There’s something very important about must and have to. The positive forms are very similar in meaning, but the negative forms are completely different.
Mustn’t = it is not allowed (non è consentito)
You mustn’t forget ... (don’t forget - you have no choice)
Don’t/Doesn’t have to = it is not necessary (non è necessario)
If you don't like him, you don't have to see him again. (there is no obligation to see him again, but you have a choice)
Here's an example you can remember: In a non-smoking area you mustn’t smoke, but in a smoking area you don’t have to smoke but you can if you want to.

5. Is have got to correct. Yes, it is
Is have got to correct? Yes, it is. In English we use both have got to, for obligation, and had better (fare bene a…, fare meglio a…, dovere), for advice, in speaking. You've got to be careful with a cat You'd better get something a bit quieter I'd better go – I mustn't miss the helicopter! So they are not used in formal writing! WARNING: even though «had better» seems to be in the past form (because of had), it is used to give advice in the present or future.

6. Should
Should is used to give advice or making suggestions about things that are a good idea to do. I can also you must for strong advice. You must go for a walk with the dog at least once a day. You must go and watch that movie. It’s fantastic! Maybe you should go for a coffee or lunch and see how you feel? You shouldn't leave it on the street. You can also use should to talk about what is likely or probable as in: He should be here in a minute = dovrebbe essere qua a minuti

7. Ought to
Ought to has the same meaning as should, but is not usually used in the negative or questions. You ought to call her = You should call her Ought to is a semi-modal verb because it is in some ways like a modal verb and in some ways like a main verb. For example, unlike modal verbs, it is followed by to, but like modal verbs, it does not change form for person: I ought to phone my parents. It ought to be easy now.

8. Ought to cannot be used with another modal verb:
Affirmative
Medicine ought to be free.
Not: Medicine ought to can be free. or Medicine can ought to be free.
Negative
The negative is formed by adding ‘not’ after ought (ought not to). It can be contracted to oughtn’t to. We don’t use don’t, doesn’t, didn’t with ought to:
We ought not to have ordered so much food.
Not: We don’t ought to have ordered so much food.
You oughtn’t to have said that about his mother.
Not: You didn’t ought to have said that about his mother.
The negative of ought to is not common. We usually use shouldn’t or should not instead: You shouldn’t speak to your father like that. (preferred to You oughtn’t to speak …)

9. Questions
The subject and ought to change position to form questions. We don’t use do, does, did:
Ought she to call the police?
Not: Does she ought to call the police?
Ought we to be more worried about the environment?
Not: Do we ought to be more worried about the
environment?
WARNING: The question form of ought to is not very common. It is very formal. We usually use should instead.