As East, you see partner lead the spade three to your ace. Partner could have three spades, but he did not raise. Besides, even if he does, what rush is there to switch? Declarer has no quick discards coming, and if you can defeat this, you will surely get another chance to attack diamonds. Meanwhile, a spade ruff might not wait if partner has led a singleton.
You select the spade eight, high for the higher side suit. You cannot afford the 10 here without setting up South’s nine.
Partner duly ruffs and shifts to a diamond, taken by dummy’s ace. Now declarer crosses to his hand with the club ace to lead a trump and … oops! Partner’s heart ace flashes onto the table, felling your king. Contract made.
Partner’s defense was perfectly logical. He was hoping you had the diamond king, in which case ducking the first heart would be fatal if South had the heart king: Declarer could cross back to hand with a club ruff and lead the spade jack for a diamond discard.
It was your responsibility to guide partner’s defense. You should have thrown your diamond queen under dummy’s ace to deny the king and bring partner into the picture. He would now be under no illusions about a diamond trick and would see no need to take the first heart.
It is a crucial precept that when leading to a trick or discarding, an honor card denies the honor above (usually showing the one below). Beware, though! The reverse tends to apply when you are following suit in second or third seat, or winning the trick in fourth seat.