| Form: must | subject + must + V; one form for every subject | She must study. |
| Form: have to | have to, but has to with he, she, it | We have to leave. / He has to leave. |
| Question, negative | have to takes do / does / did and then the base form | Do you have to leave? / She doesn't have to leave. |
| must | the speaker's own view or a written rule | I must save more money. / Visitors must sign in. |
| have to | the situation or another person decides; common in speech | I have to get up at six for work. |
| mustn't | forbidden | You mustn't use your phone here. |
| don't have to | not necessary: you can choose | You don't have to come. |
| Trap | had to is the past of must | I had to work on Saturday. |