I have seen uses of must that appear to be in the simple past tense. Sometimes these seem grammatical, but sometimes not. Examples that help illustrate my confusion:
He knew he must go to New York - sounds fine to me.
He went to New York because he must - sounds questionable.
Because he must go to New York, he bought plane tickets - sounds completely ungrammatical.
He must go to New York - grammatical, but with no other verbs to provide context, can't interpret this as being past-tense.
My question: is this past-tense use of must ever acceptable? Is it only acceptable in certain contexts - if so, what are those contexts and why?
(Note - I'm not looking for had to. Have is a different verb, so had to expresses the same meaning as the past tense of must, but it itself is not the past tense of must.)