Must (expressing obligation)
Only 1 form: Must (e.g. no past tense, no perfect tense);
Use:
a) When something is obliged (personally or from a source outside) or not allowed:
( My mother told me that ) I must wear sunscreen or I will get a sunburn. (personal obligation and an obligation from a source outside)
You must not park your bike in front of the school. (it is forbidden)