Both of the following are correct: - He left before the sun set. [= He left before the sun went below the horizon. ] - He left before sunset. [Sunset = the time the sun went below the horizon.]

You could also say, "He left the place before the sun had set."

To set is a verb. The sun did this action. The sun set. It is correct. Sunset is the time when the sun sets, just as dawn is when the sun rises.

According to me "He left the place before the sun set " is absolutely correct ...because "sun" is universal and every universal word is allowed by an article 'the'

Each gives a different interpretation. Before sunset implies more of a general time, while before the sun set focuses more on the exact moment. Sunset sounds better in general, but if you want to emphasize the moment, then before the sun set works