Take the South hand as declarer in 4♠. West leads the trump ace and switches to the ♦3. Which diamond do you play from dummy and why?
Count your tricks (this is a recording). You have five spades and three hearts for eight, so you need two more. If East has both the ♦A and the ♦Q, you’re toast (you’ll lose two diamonds and the major-suit aces), so assume West has one of those cards. That means you’ll score a diamond trick eventually. The game-going 10th trick, therefore, must come from the ♣K, and the only way that can happen is if West has the ♣A.
West has already turned up with the ♠A, and you need him to hold the ♣A, too, so does he have the ♦A as well? These days, not many folks will pass holding three aces, so if you’re going to assume that West has the ♣A, you shouldn’t play him for the ♦A, too. Play him for the ♦Q instead and call for dummy’s ♦J.
You might very well go down on this deal, but you should at least try to place the missing high cards in a manner consistent with the auction.