Obligations, Prohibitions and something else

Weak obligation is often described in grammar books as the right/ correct thing to do. Weak obligations may come from tradition, custom or culture and may be seen as advice. They are also use to express suggestions, recommendations or even crtics regarding an issue or situation.

Our friend is unwell. We should visit him.

You ought to prepare your bags the night before your flight

Careful there! You're driving. You should pay more attention.

It is used for specific obligations or obligations that are self imposed. This word is used only to express present obligations. To express past or future obligations it is only used "had to" or "will have to".

You must come.

Must I wear this tie?

My tooth is sore. I must go to the dentist.

it mainly expresses general obligations. It is associated with rules stated by the government, organizations or institutions.

I have to brush my teeth twice a day.

Do I have to wear this tie?

To travel, you have to carry a passport.

This one expresses prohibition. It is consider a strong obligation not to do an action. In this sence, this word can be changed by the expression "notbe allowed". The latest is the one to be used when expressing prohibitons in the past or in future.

You mustn't drive on the sidewalk or across the park.

You mustn't take your cellphone out when in the bank.

You mustn't bring food inside the classroom.

This one expresses the absence of obligation or necessity. This gives the idea that a determine way to act is optional and is only under the consideration of the doer.

You don't have to drive if you don't feel like doing it. I can do it for you.

She doesn't have to wear a dress for the party but she would look great in it.

It is used to indicate that someone did something,, but it was not necessary nor requested though well received. This form is used only to indicate past events.

You needn't have washed the dishes. I would've put them in the dishwasher.

You needn't have prepared all that food. They phoned to say they wouldn't be coming.

Pay attention to the following sentences and choose the idea they're transmiting.

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