Both Polonius and Laertes give advice to Ophelia in Act 1, Scene 3. Laertes enjoins his sister not to take any expressions of love from Hamlet seriously and to protect her virtue. In other words, modern words, Laertes tells his sister that he believes Hamlet only says words of love to get Ophelia in a loving mood and go to bed with him.
Polonius agrees with his son, Laertes, but with a slightly ulterior motive. He does not want his daughter to dishonor the family name by having a affair with Hamlet when he is sure the prince does not want to make the union an honorable one by marrying Ophelia.
In Act I, scene iii, Laertes warns his sister to not take Prince Hamlet's profession of love seriously as marriages for princes are usually matters of state. In other words, royal marriages are made for political reasons, not for love:
...Perhaps he loves you now,
...but you must fear,
His greatness weighted, his will is not his own,
Also, Laertes warns Ophelia against to restrain her affections lest she be embarrassed: "Be wary then..." Ophelia promises to heed his advice.
Polonius enters and bids Laertes be prudent on his venture to Paris. Then he asks his daughter about Hamlet and her. When she says Hamlet has been affectionate, Polonius cautions her as did Laertes: "Tender yourself more dearly...or you'll tender me a fool." To this Ophelia replies that Hamlet has been honorable. But Polonius is skeptical:
"When the blood burns, how prodigal the soul...From this time
Be something scanter of your maiden presence."
Polonius warns his daughter against showing affection, also. However, his reasons are that Hamlet may simply be trying to seduce her with "mere implications of unholy suits.
To both her brother's and to her father's advice Ophelia promises to "abide."