Вивчаючи статистику

UNIT 9. Modals
Modals can/could, may/might, must/[had to], ought to, shall/should, will/would:

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  don't take -s, -ing or -ed suffixes
  
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  are followed by the bare infinitive
  
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  come before the subject in questions and are followed by not in negations
  
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  don't have tenses in the normal sense. When followed by a normal bare infinitive, they refer to an uncompleted action or state (i.e. present or future); when followed by the bare perfect infinitive, they refer to a completed action or state.
  

Obligation/Duty/Necessity
Must: Expresses duty/strong obligation to do sth, shows that sth is essential. We generally use must when the speaker has decided that sth is necessary (i.e. subjective).
I must remember to send my mother a birthday card.
Have to: Expresses strong necessity/obligation. We usually use have to when somebody other than the speaker has decided that sth is necessary (i.e. objective). He said we have to follow the instructions.
Had to is the past form of both must and have to.
Absence of necessity
Don't have to: It isn't necessary to do sth in the present/future.
You don't have to do the washing up-1 will do it.
Didn't have to: It wasn't necessary to do sth. We don't know if it was done or not. He didn't have to pay all the bills today. (We don't know if he paid them or not.)
Permission/Prohibition
Can/May: Used to ask for/give permission. May is more formal than can.
Can/May I borrow your pen, please? Yes, of course you can/may.
Mustn't/Can't: It is forbidden to do sth; it is against the rules/law; you are not allowed to do sth.
You mustn't/can't park your car here.
Possibility
Can + present infinitive: General/theoretical possibility. Not usually used for a specific situation.
For dessert you can have apple-pie or ice-cream.