"...by my advice, Let us impart what we have seen tonight Unto young Hamlet." (i.i.184-186)
The guards of the castle are trading shifts at night when they see the ghost of King Hamlet and decide to tell the Prince.
"Thaw, and resolve itself into a dew, Or that the Everlasting had not fixed His canon 'gainst self-slaughter! O God, God!" (I.I.130-132).
Gertrude has married Claudius while Hamlet is still grieving. He is brought to the brink of suicide in his depression. Horatio tells Hamlet about the ghost of his father.
"For Hamlet and the trifling of his favor, Hold it a fashion and a toy in blood, A violet in the youth of primy nature, Forward, not permanent, sweet, not lasting, The perfume and suppliance of a minute. No more." (I.III.5-10)
Laertes tells Ophelia not to fall in love with Hamlet and Polonius agrees. Polonius tells Laerets how he should behave while he is at school.
"My fate cries out And makes each petty artery in this body As hardy as the Nemean lion’s nerve. Still am I called.—Unhand me, gentlemen. (draws his sword) By heaven, I’ll make a ghost of him that lets me. I say, away!—Go on. I’ll follow thee" (I.4.86-92).
Hamlet and his friends wait for the ghost to appear at night and when it does Hamlet chases it .
"'Tis given out that, sleeping in my orchard, A serpent stung me. So the whole ear of Denmark Is by a forgèd process of my death Rankly abused. But know, thou noble youth, The serpent that did sting thy father’s life Now wears his crown" (I.5.35-40).
The ghost of the late King Hamlet tells the prince that he was murdered by Claudius via poison and he asks the prince to avenge his death. After the ghost disappears, Hamlet asks his friends to keep his secret.