No, you can't say 'must to'. 'Must' is a modal verb, like 'can' and 'may', so the verb comes directly after it. You can say 'I must go' or 'I have to go' (but not 'I must to go'). 'Must' is an alternative to 'have to' in affirmative sentences in the present simple tense only. 1. In negative sentences, you need to use 'have to' . The negative of 'I must go' is 'I don't have to go'. 'I don't have to go' = It isn't necessary for me to go. (NB 'I mustn't' has a different meaning = I'm not allowed to go) 2. In all other tenses - perfect, past, future etc, you need to use 'have to'.

Interchangeable, but "Have to" is colloquial (at least in American English). I must go. (Sounds like poetry or old English) I have to go. (What you say with your family, friends, co-workers).