Английская грамматика: модальные глаголы

3. When we are mentioning someone else's obligations, we use have to.
John has to quit working in this company.

4. For questions it is more common to use have to instead of must (which sounds very formal):
When do you have to finish the report?
5. The past tense of must is had to:
I had to pay my bill yesterday.

Mustn't and Don't have to:

Mustn't is a negative obligation (= it is important that you do NOT do something) while don't have to is an absence of obligation.
Mustn't = it is prohibited; it is not allowed
Don't have to = no obligation; you are not required to do something, especially if you don't want to.
You must not drink this water. (= it is forbidden to drink that; it is not allowed)
You don't have to drink this juice. (= you don't need to drink that but you can if you want)

4. Modal verbs. Should

Теория:
Study the information about should.

After should you use the base form of the infinitive.

Should + Verb (base form of infinitive)
You should go now.
Should is used in these situations:

1. To give advice, a recommendation or a suggestion.
This is to say that it is the right thing to do or the correct thing.
You should make an appointment with the dentist.

2. Expresses that a situation is likely in the present.
He should have the letter by now. I sent it three weeks ago.

3. Expresses that a situation is likely in the future (prediction).
I posted the cheque yesterday so it should arrive this week.

4. Expresses an obligation that is not as strong as must.
Sometimes should is used instead of must to make rules, orders or instructions sound more polite. This may appear more frequently on formal notices or on information sheets.
You should be at work before 9.