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Must and have to both say that an action is necessary: You must switch off the lights. / We have to finish by six. Must is common in written rules and notices; have to is common when we talk about our duties. Both are followed by the base verb: no to after must and no -s on the verb. Must never changes, but have to does: I have to, she has to. Typical mistake: She must to call the client. → She must call the client.
Mustn't and don't have to look similar, but they mean very different things. You mustn't park here = it is prohibited, so do not do it. You don't have to park here = it is optional, so you can choose another place. With he, she or it, use doesn't have to: Maria doesn't have to wear a uniform. Typical mistake: using don't have to for a prohibition. For the rule about smoking, say You mustn't smoke in the office, not You don't have to smoke in the office.