You will often be in contracts you and your partner bid perfectly; you're at the right level in the right denomination; but the contract fails. Have you made a mistake by being in this contract? Not at all. The fact that a particular contract fails does not mean that you should necessarily have been in a different contract. All it means is that the cards were in the wrong place at the wrong time. 'Tis unfair and unkind I know but it's the nature of the beast I'm afraid.
Most times, I can tell you what contract you SHOULD be in, but I can't possibly tell you whether that contract WILL make or not - because I don't know how the suits are split, or where the outstanding honours are. This is exactly why the game is so fascinating: nothing is certain. The likelihood of making various contracts is not cast-iron but dependent on numerous factors outside your control. All you are trying to do is make some sense of the random deals you are dealt. If 30% of the contracts you bid fail to make, this is not because you are a bad player, but because the game is only about possibilities. And if you are making 70% of your contracts, you're either very good or you're not bidding high enough!
If you play in a suit contract with an 8 card fit between you and your partner, then opponents hold 5 cards of your suit. Let's say that you can make the contract, whatever else happens, if those 5 cards split 3- 2. Well bid, well played.
But now assume those cards are split 4 - 1. Now in order to make the contract you might need certain cards to be well placed, for finesses to work, for endplays to be carried out, or failing all else, for the defenders to get it wrong and let you make it when you shouldn't. Now, well bid and even better played. OK.
What if the cards are now split 5 - 0? Ouch, I hear you proclaim. Now your contract is almost certainly doomed. You can't control the trump suit and you probably can't establish a side suit without them trumping your winners. How unkind, how unfair. But nothing has changed about the nature of you and your partner's hands, or your bidding. It's just that every now and then it will all go tits up - guaranteed. 100% certainty.
It helps to have a couple of little gadgets up your sleeve as well. Here we have the HOLD UP and the DANGER HAND. You're in 3NT (everyone's favourite contract!):
The bidding goes 1D-1S-1NT-3NT.
West leads the king of hearts. If you take this with the ace you are in serious danger of going down when you get the diamond finesse wrong and lose four or five hearts and the queen of diamonds. So you must NOT win the ace. Yet.
You can see 3 spade tricks, 1 heart, 2 diamonds and 2 clubs - 8 tricks. You need one more and that can only come from the diamond suit. You can guess to finesse the queen either way. You get lucky, you make maybe ten tricks. But if you guess wrong, you're down. So don't guess. Think.
You must HOLD UP the ace until forced to play it at the third trick. On this trick East, on your right, discards a club. This means that he had two hearts and that therefore West started with six hearts. If West gets back in you will lose three more tricks in this suit to add to the two the opponents already have. West is the DANGER HAND! You must not let West back on lead so you cannot, must not finesse the ten of diamonds.
Now there is no guess. You win the ace of hearts at Trick 3, cash the king of diamonds in the unlikely - but possible - event of the queen of diamonds being single, and play a low diamond to the jack. If it wins you're home and dry. If it loses you're still home and dry. Why? Because East hasn't got any more Hearts. Nor, on this layout, can East put his partner back on lead quickly enough to damage your contract.
There's a lot of versions of both these fairly simple ploys, so keep an eye out for them,