Modal Verbs: Obligation

We use 'must' and 'have to' to talk about obligation, but they feel a little different. We use 'must' for a strong, personal obligation, when the speaker feels something is very important (I must call my mother today). We use 'have to' for an obligation that comes from outside, like a rule, a law or another person (Students have to wear a uniform; it's a school rule). In the third person 'have to' changes to 'has to' (She has to get up early), and in the past both 'must' and 'have to' become 'had to' (Yesterday I had to study late). Be very careful with the negatives, because they mean different things: 'mustn't' means something is forbidden or not allowed (You mustn't smoke here), while 'don't have to' / 'doesn't have to' means it is not necessary, but you can do it if you want (It's Sunday, so we don't have to go to school). After 'must', 'have to' and 'has to' we use the bare infinitive (You must wear a seatbelt). For example: 'You don't have to bring food, but you mustn't bring your phone into the exam room.'