How does Hamlet respond to Ophelia at the beginning of Act 3?
In Act III, scene i of Hamlet, Hamlet appears to Ophelia after Ophelia, the king and queen, Rosencrantz and Guildenstern, and Polonius discuss Hamlet's seeming descent into madness. Gertrude tells Ophelia that she hopes that Ophelia and Hamlet will come back together as they were before, which makes Hamlet's response to Ophelia moments later even more difficult for Ophelia to understand.
Hamlet's response to Ophelia when she asks him, "How does your honor for this many a day?" is full of strangeness—all the more strange for her simple and polite question. Ophelia mentions some things that Hamlet has given her, and he responds by contradicting her and saying that he never gave her anything. He then asks a series of odd questions about Ophelia's beauty and goodness—rhetorical questions that he doesn't expect Ophelia to answer—and soon he tells Ophelia "Get thee to a nunnery" after criticizing her for unreliable qualities she does not actually exhibit.
These responses all combine to confuse and alarm Ophelia. She feels sure that Hamlet has gone insane, and when he exits, she says, "Oh, what a noble mind is here o'erthrown!"