The past tense of have to is had to, with did and didn’t in the question and the negative.

• I had to get up early this morning.
• Why did you have to work last weekend?
• They liked the hotel because they didn’t have to do any cooking.

Have to expresses strong obligation (silna povinnost). The obligation comes from ‘outside’ perhaps a law, a rule at school or work, or someone in authority.

• You have to have a driving licence if you want to drive a car.
• I have to start work at 8.00.
• The doctor says I have to do more exercises.
• Don’t/doesn’t have to expresses absence of obligation (it isn’t necessary).
• You don’t have to do the washing up. I’ve got a dishwasher.
• She doesn’t have to work on Mondays.

Must is also used to express strong obligation. Generally, when it is used, the obligation comes from the speaker.

• I must get my hair cut.
• This suggests that I feel it is necessary.
• You must … can be used to express a strong suggestion.
• You must see the Monet exhibition! It’s wonderful.
• You must give me a ring when you’re next in town.

The following are modal auxiliary verbs: can / could / might / must, shall / should / will / would. They help another verb. The verb is the infinitive (without to).

• She can drive.
• I must get my hair cut.
• I must to go.
• You should tell the truth.
• Wrong: I can to swim
• Right I can swim.
• I must go.
• There is no do/does in the question.
• Can she type?
• Should I go home?
• Wrong: Do you can type?
• Right: Can you type?

The form is the same for all persons. There is no –s in the third person singular.

• He can dance very well.
• She should try harder.
• It will rain soon.
• Wrong: He cans dance.
• Right: He can dance.

To form the negative, add n’t. There is no don’t/doesn’t.

• I can’t spell.
• I wouldn’t like to be a teacher.
• You mustn’t steal.
• WrongI don’t can help you.
• RightI can’t help you.

Most modal verbs refer to the present and future. Only can has a past tense form, could.

• I could swim when I was three.

The forms of should are the same for all persons.

Positive and negative

Question

Short answers

Should I phone home? Should I buy a Mercedes-Benz?
Yes, you should. No, you shouldn’t.

Should is used to express what the speaker thinks is right or the best thing to do. It expresses mild obligation, or advice.
I should do more work. (this is my opinion)
You should do more work. |(I’m telling you what I think.)
Do you think we should stop here? (I’m asking you for your opinion.)
Shouldn’t expresses negative advice.
You shouldn’t sit so close to the TV. It’s bad for your eyes.
Should expresses the opinion of the speaker, and it is often introduced by I think or I don’t think.
I think politicians should listen more.
I don’t think people should get married until they are 21..