Hi Mike! I know this question has been asked before by someone else but I am still confused: (To stand in) Persepolis in modern Iran and look out, as Darius the First (must have done), at the immense (sweep of fields and mountains) is (to grasp) the vastness of the ancient Persian Empire. (No error)
When I looked in a grammar book that I had, I found out that “must” is in present tense and “had to” is the correct past tense. Since Darius the First clearly belongs to the past, I’m very confused.

General rule: if a question has already been asked, please ask your question as a comment on that post rather than resubmitting. That way, future people searching the question will find one post with all the discussion, and I don’t end up repeating myself too often.

“Must” isn’t by itself here. The verb is “must have done,” which is definitely in the past tense.