Marcellus thinks that Horatio may know how to speak to a ghost because he is a University Man. I guess Marcellus assumes that's the kind of thing you learn at a University.
Yes the ghost only speaks to Hamlet. The others can see him but cannot hear him. When the ghost visits Hamlet in his mother's closet, his mother can neither see nor hear him.
His father's ghost appears to him in a grave yard and tells him who and how he died.
Hamlet's dead father
Marcellus and Barnardo
to see if its really a ghost
Marcellus and Barnardo assume that Horatio, being an educated man, will have studied Speaking to Ghosts 101 at Wittenberg University and will know how to talk to it.
Horatio tell HAmlet that after midnight his father ghost shows up
Marcellus asks Horatio to watch with them because he believes Horatio is a rational and educated man who can provide a credible perspective on the ghost they have seen. He hopes that Horatio, being skeptical and grounded, will help validate their experience and discern the ghost's intentions. Additionally, Marcellus values Horatio's judgment and wants him to witness the supernatural event firsthand.
names the chapters that speak only on the holy ghost in the bible?
The Queen can't see him. And that is really odd, because when the ghost appeared on the battlements, everyone could see him: Horatio, Marcellus and everyone.
"Never to speak of this that you have heard", by which he means the appearance of the ghost, although Horatio heard nothing from the ghost's lips. Also, "How strange or odd soe'er I bear myself . . .that you, at such times seeing me, never shall . . . note that you know aught of me." Horatio and Marcellus are not to suggest even that they have an idea why Hamlet is doing what he is doing.