‘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.’

The sentence contains no errors; however, I am slightly confused. Isn’t the word ‘done’redundant? How do I know if a word is redundant or not? Thanks!

“Done” is not redundant. You could replace “done” with “stood,” or just remove it, if you like, but those would just be style choices. All are equally correct.

Try to resist going overboard looking for redundancy. It’s tested very rarely, and when it is, it’s usually obvious (e.g. “annually” and “every year” in the same sentence).