Books about Bridge will introduce players to how they should bid by explaining the basic principle and by giving a few examples. To make the principle easier to understand, though, it would seem that it would be helpful to give a larger group of examples, even if they include some less likely to occur in practice.
For example, if you hold the Ace of the trump suit, you are guaranteed to win one trick, because it is the most valuable card in the deck. If you hold both the King and Queen of the trump suit, you are also guaranteed to win one trick, because there is only one card, the Ace, that can beat either of them that someone else might hold.
Holding both the King and Queen is slightly better, though, because it might happen that your partner is the player holding the Ace, or that it might be used up in the game for some other reason. It could be that it is for that reason that a more extensive table of possibilities where one trick is forced isn't usually given in books for beginners, because it is somewhat misleading as to the relative values of the hands.
Cards in a suit that force one trick as trump:
A K Q Q J 10 J 10 9 8 10 9 8 7 6 9 8 7 6 5 4 8 7 6 5 4 3 2
These aren't the only possibilities. If you hold K J 10 instead of Q J 10, you will also be able to force only one trick, for example. So the table above shows the lowest cards you can have, for a given number of cards, to force one trick.
And here are cards in a suit that would force two tricks as trump:
A K K Q J Q J 10 9 J 10 9 8 7 10 9 8 7 6 5 9 8 7 6 5 4 3
Any combination of three cards that is as high as Q, J, 10, but not as high as K, Q, J can force one trick.
So the principle of counting forced tricks is to count the cards in a suit that you do have, versus the ones you don't have, from Ace down to 2. If, at any point, the number of cards you do have, in the ones you have counted so far, is greater than the ones you don't have, the hand can force the win of at least as many tricks as the difference between the two numbers is, so the highest value this reaches during the count is the value of that suit.
The winner of any trick leads to the next trick, and thus a goal is to play one's guaranteed tricks first, so that one retains control of the suit that is led. Since you have to follow suit if you can, and only a trump card or a card in the suit led can win a trick, a good card in the wrong suit at the wrong time is wasted.
Copyright (c) 2008 John J. G. Savard