Modal Verbs

The sentence 'We don't must to go swimming' is grammatically incorrect. The correct form would be 'We don't have to go swimming.'

Must is used to express a strong obligation or necessity.

Should is the standard modal verb used for giving advice or making recommendations.

Shouldn't is the contraction of should not, which is used to advise against an action or state that something is not a good idea. Must not and mustn't express prohibition, while don't have to expresses a lack of obligation.

Should is used to express that something is a good idea, recommended, or the right thing to do. Have to and must express obligation, while don't have to expresses a lack of obligation.

The phrase has to indicates an obligation or necessity. Since the sister is sick, it is necessary for her to stay in bed, making the statement true.

Don't have to is used to indicate that an action is not necessary or that there is no obligation to perform it. Don't need to is also used for this purpose, but don't have to is the standard form matching the provided options.

Have to is used to express that an action is necessary or required. While need to also expresses necessity, have to is the intended answer for this specific grammar exercise.