Answer:
Bernardo was on guard, guarding the castle.
Step-by-step explanation:
William Shakespeare's "Hamlet" revolves around the revenge plan of a young prince for the death of his father, the king. The play delves into the themes of love, loyalty, greed, power, insanity, women, etc.
In the first scene of Act I, two "sentinels" are seen on stage. One of them is Bernardo, who had come to relieve Francisco for the guard duty. Also, we know he's there for the guard duty from his line "If you happen to see Horatio and Marcellus, who are supposed to stand guard with me tonight, tell them to hurry."
Thus, Bernardo was on guard.