* We use have to and need to for expressing obligation or necessity. For example: "I have to go to work today."
* We also use must for expressing obligation, it is more formal. For example: "I must do my homework."
* In the negative, must not is used for expressing prohibition. For example: "You must not smoke in the hospital."
* In the negative, don't have to and don't need to are used for expressing lack of obligation. This means you have a choice. For example: "You don't have to drink beer, we have wine if you want it!"
* should is used for expressing advice or recommendations in the positive and negative. For example: "You shouldn't eat so much salt!"
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