Enters York Lo, the young historian. (Julian Suez)
What does the quoted sentence mean? What is the subject of the sentence?
It’s just a stylized way of saying “York Lo, the young historian, enters.” It mimics the script of a play and highlights this as sort of a “triumphal entry”, like York Lo is going to save the day, or at least try to.
Enter Ariel. (The Tempest)
http://www.opensourceshakespeare.org/views/plays/characters/charlines.php?CharID=Ariel&WorkID=tempest&cues=1
What is the subject of the above sentence?
Why didn’t Shakespeare write “Enters Ariel”?
Maybe now in our enlightened times we have better grammar than poor Shakespeare, so “enters” instead of “enter”
But seriously, you are correct, most stage directions, including Shakespeare’s, are given in the imperative mood, so the actual subject is “you” - “-You- enter, Ariel”.
The stage director usually sort of prompts like this to the character so as to avoid any pause between two scenes. So, it is as if he says it in the imperative form: You, Ariel, enter now. (Enter, Ariel) The subject is the implied You. The punctuation (comma) was not so significant a grammatical aspect in Shakespearean era because “we have better grammar than poor Shakespeare” as rightly observed by Luschen.