Second Answer
Against three notrump, partner leads the nine of spades (four, Jack, five). You return the three of spades (six, eight, King). Clearly, declarer has the Ace and ten of spades remaining, perhaps another spade as well. Next, declarer leads a club to the Ace, a club to the King (you discard the five of diamonds), and the Queen of clubs (you discard the six of diamonds, declarer follows, and partner follows). This position remains, with declarer about to cash three club winners from dummy:
Your first discard will be the ten of diamonds (declarer discards the three of hearts, partner the deuce of spades). What will be your other two discards? Two spades, one spade and one heart, or two hearts? Declarer still has the Ace and ten of spades, so you have to keep both of your spades, and you will discard two hearts.
Bonus from Val: don’t worry about bad things that might happen as a result of discarding hearts. You know it will be fatal to discard a spade, but it’s possible that you can afford to discard the hearts.