​Obligation and necessity

​Obligation: must or have to;

"have to" - an obligation we see as outside our control - rules imposed by authority;

e.g.: Students have to wear uniforms.

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"must to" - ​obligation imposed by the speaker;

e.g.: I must get up earlier. ​

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Must used for:

• to give a personal opinion: I believe people must vote.

• to impose an obligation on oneself: I really must lose some weight.

• to give instructions: The electricity must always be switched off before repairs.​

​!!!! "Must" - a strong obligation and we do not often use it in face-to-face conversation and we often use "should" or "ought to".

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• "Need" - external obligation;

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• "should/ought to + have + past participle" - unfulfilled obligation in the past:

e.g.: You really should have/ought to have r​egistered before term started.

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• ​"need to, must and have to" - express necessity​;

e.g.: I really need to/must/have to get some more sleep.​

​Prohibition and criticism

• " must to" - prohibition imposed by the speaker;

e.g.: You mustn't touch that kettle! It is hot!

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• ​- "shouldn't/oughtn't to" - weaker, moral prohibition;​

e.g.: You shouldn't/oughtn't to speak to your mother like that!

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To express prohibition by an external authority we often use other modal verbs such as "may not" , "can't" , "aren't allowed to".

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​Absence of obligation or necessity

​"don't have to/don't need to" - lack of external obligation;

e.g. You don't have to pay to visit the museums in Britain.

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"needn't/don't need to" - absence of necessity;

​e.g.: You don't need to put the heating on.

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!!! "Need" has two past forms: "didn't need to" and "needn't have done"​.

​Recommendation and advice

• "must" - strong recommendation and advice​

e.g.: You must do something about that cough.

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• "should and ought to" - used with the passive;

e.g.: The underlying shift in public opinion ought no to be exaggerated.

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"had better" - expresses the best thing to do in a particular situation.

e.g.: Your dog had better not dig up my rose bush again!​

​Logical deduction and probability

​"must" - for something that we believe to be true because of evidence;

​e.g.: This must be the place - it is the only restaurant in the street.

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​Opposite here is can't, NOT mustn't!!

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"must have + past participle" - logical deduction about the past;

e.g.: There was a terrific noise last night. It mush have been an explosion.​

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Negative here is : can't/couldn't have + past participle!​

"should/ought to" for probability and the negative for improbability.

e.g.: The plane should be landing about now. ​

"should/ought to + have + past participle" - probability of an action in the past.

e.g: He should have arrived hours ago. ​