As South, you hold:
You reach 4 as shown with the K lead. What do you play from dummy?
You correctly let the opponents win the first trick.
West shifts to a trump to East's ace. East plays a second trump as shown below.
I don't know. You're the teacher and you got me into this position.
Win and try to set up hearts.
Win and draw trump.
Trumps are 3-2. You have lost Trick 1 and the A. You still have 2 losing diamonds in your hand.
Accordingly, you need to do something with dummy's hearts.
Your good planning means that the A still remains as an entry.
So, you lay down the A and see:
Next you play to the K and see:
When you play the 9, East discards a club. Now what?
Trump it and go down (hearts were unfortunately 4-2).
Discard a diamond.