He warns her to watch out for Hamlet, because his protestations of love may be merely designed to seduce her.
In Act 1, Scene 3 of Shakespeare's Hamlet, Laertes warns his sister, Ophelia, to be cautious of her relationship with Prince Hamlet because his intentions may not be honorable and he fears she could be hurt in the process. He advises her to guard her virtue and not give in to Hamlet's advances.
Laertes, in a parallel revenge, seeks revenge against Hamlet for killing his father, Polonius, and the early death of his sister Ophelia. Hamlet does die, but not before Laertes, so Laertes never does get his revenge against Hamlet
It shows most dramatically in Scene 16, (Act 4 scene 5,) when Laertes leads a mob to storm the castle.
It illustrates Laertes close relationship to Ophelia.
Laertes accuses Claudius of killing his father Polonius in Act IV, Scene V.
Laertes.
To France, so he can spy on Laertes.
I can't give you a response unless I know what he is supposed to be responding to. "Thanks Rosencrantz and gentle Guildenstern" is a response to what those two guys tell him at the beginning of Act III Scene I for example..
Hamlet to Laertes, who's choking him in his sister's freshly dug grave: "I prithee take they fingers from my throat." Hamlet naturally picked the least appropriate time possible to be polite.
The crowd that enters with Laertes wants to seek justice and avenge the death of Polonius, Laertes' father. Fueled by anger and a sense of betrayal, they rally around Laertes, demanding action against those they hold responsible, particularly Hamlet. Their presence adds to the tension and chaos in the scene, reflecting themes of revenge and mob mentality.
In the last scene of the play, before Hamlet and Laertes are to have their swordfighting match, Hamlet makes a prepared speech of apology beginning with the words "Give me your pardon, sir. I've done you wrond, but pardon it as you are a gentleman." He goes on to say that if he ever did anything to offend Laertes, it was something he did when he was crazy. It's a convenient excuse which nobody buys.
The scene may be set at the harbor of the local town of Elsinore, as in the BBC production with Derek Jacobi. Laertes says that his luggage (his "necessaries") are embarked, and Polonius notes that the wind is right for sailing and commands Laertes to go aboard. The harbor setting gives an immediacy to these comments. On the other hand, many productions set this scene indoors where it would be more appropriate for Laertes and Ophelia to be discussing her sex life. Nothing in the script explicitly says that the ship is actually present where they have their leavetaking.