Must is a modal verb. Modal verbs do not change in form to make different tenses.

We say "you must do [x]" to mean that you should do that thing in the future. Looking back, you can only say if they did or did not do it. If someone didn't do something they were told they must do, we would say they should have done it, but that is not a different participle of the same verb because nothing has actually been done.

Must is a modal verb. Modal verbs do not change in form to make different tenses. Both your examples are fine, but they are using different verbs. 'Must' and 'had to' are not irregular forms of the same verb.

Of course, as auxiliary verbs, modals are used alongside another verb which most likely will have different forms. So, for example, if you say "you should go", retrospectively you might say "you should have gone".

'Must' is used differently to 'should'. We use 'should' for actions or responsibilities that we think are best for the concerned subject. We use 'must' things that are considered *compulsory or necessary. For this reason, we don't use 'must' to say that someone ought to have done something in the past - we would usually just say that they should have done it to fulfil their compulsory obligation. When we do use 'must' about something in the past it is to express disbelief that it wasn't done.

Must is a modal verb. Modal verbs do not change in form to make different tenses.

We say "you must do [x]" to mean that you should do that thing in the future. Looking back, you can only say if they did or did not do it.

Must is a modal verb. Modal verbs do not change in form to make different tenses.

We say "you must do [x]" to mean that you should do that thing in the future. Looking back, you can only say if they did or did not do it. If someone didn't do something they were told they must do, we would say they should have done it, but that is not a different participle of the same verb because nothing has actually been done.