Playing and defending no trump is the most challenging part of bridge. It requires planning and doing things that are sometimes counter-intuitive. Here's today's hand:
South West North East
P 1S P 2H
P 2N P 3N
P P P
Opening lead: 3C
First the bidding: When East reverses (bidding a higher ranking suit at the two level than the suit with which you opened the bidding at the one level; a "reverse" promises at least 17 HCP and is forcing for one round) by bidding two Hearts after her one Diamond opener, West is forced to bid again, so chooses the weakest response possible, considering that he has the unbid suit, Clubs, stopped. East likes her hand and raises to game.
Superficially, it looks like you can take three spade tricks, two or three diamonds, and a heart or two, plus one Club. But closer inspection shows that this is all dependent on transportation. You have to get rid of the Ace of Spades on the board before you take your King and Queen in your hand. Plus you have to take two diamond finesses for this to work. But your hand is woefully weak and the singleton Ace of Spades doesn't help transportation since your only real entries are spades. How are you going to get to your hand enough times to accomplish all this?
You take the first trick in your hand with the Club Jack. Then you take your first diamond finesse. If the honors are split, you can take three diamonds. When South wins the Queen of diamonds, she returns a spade to the board's singleton Ace. You now have to play South for the Ace and Jack of Hearts to get back to your hand. So you lead the Heart 6 to your Ten. South takes her Jack. Recognizing the transportation problems and wanting to keep the lead on the board, South takes her Ace of Hearts, figuring she'd then put the lead back on the board (that would leave the KQ of Hearts on the board) and you'd be stuck with no way to get to your hand to take your two spades and the second diamond finesse.
Here is the key to the hand. You discard your King of Hearts on her Ace, so when she leads another heart, you are able to take the trick in your hand with your Heart Ten. Since you discarded your King from the board, you have the eight to play from the board to take the trick with the Ten in your hand (if you had discarded the eight on her lead of the Ace, you are left with just the King and the Queen on the board and you're blocked from returning the lead to your hand). That allows you to take the two spade tricks in your hand and the second diamond finesse, which works, and the King falls when you play the Ace, the diamonds splitting favorably, giving you your three diamond tricks. So you end up taking three spades, three diamonds, two hearts and a club, making three. If you do not discard your king of hearts on her Ace, the hand can't make, assuming a reasonable defense.