Let’s review the common ways to express obligations and prohibitions in English
•Have to is used to talk about external obligation, which means that the obligation does not come from us but from other person or authority.
👉 I have to study (because my parents said so)
👉 They have to finish the project by Friday (because their boss asked for it)
•Not have to is used to talk about that things that are optional (no tienes por qué hacerlo)
👉 You don’t have to clean the dishes (it is not necessary, I’ll do it later)
👉 She doesn’t have to help me. I can do it. (it is optional)
•Must is used to talk about internal obligation, which means that the obligation comes from us. It can also be used to talk about rules that are mandatory
👉 I must exercise a bit (otherwise I’ll get fat and I don’t want that)
👉 All passengers must remain seated after landing (it’s the airline’s rule and we need to obey it)
•Mustn’t is used to talk about prohibitions or things we need to avoid doing
👉 You mustn’t smoke in here (it’s forbidden)
👉 She mustn’t eat so much chocolate (or she’ll get sick!)