If you hold Ace and others in a plain suit, partner leading Jack, pass it if Dummy has no honour. Perhaps by winning the second round you can give the invited force. With any other honours than the Ace, pass a partner’s Jack led.
If partner leads you a suit of which he knows, or should know, you have not the best, he must have a good finesse in the suit which he does not lead, and you should take the first opportunity to lead that suit to him.
In returning partner’s suits, some modification may be suggested by the condition of Dummy’s hand. For instance: With K x x; Dummy having A Q J x; if you win, third hand, on Dummy’s finesse, you may be sure your partner’s lead was a weak suit. If Dummy is weak in the two other plain suits, your partner may have a good finesse in one or both of them.
When your partner wins the first round of an adverse suit, and immediately returns it, he is inviting a force.
Dummy on the Left. When the player is third hand with Dummy on his left, his chief care will be to divine his partner’s object in leading certain cards up to Dummy.
The general principles of inference are the same as in the preceding case, and cards may often be inferred in the same manner from the evident intention of partner. For instance: You hold K x x; partner leads J, declarer covering with Queen. A glance at Dummy’s cards shows him to have 10 x x; so your partner may be credited with A 9. You have x x; your partner leading Q, covered by declarer with K, and Dummy having J x x. You may credit your partner with A 10. You have x x; your partner leads Q and declarer wins with Ace; Dummy holding 10 x x. Your partner must have J 9 and others, and the declarer has the King.
There are several cases in which you should not allow Dummy to win the trick. If you have only one card of a suit in which your partner leads Ace then Queen, and Dummy has the King twice guarded, trump at once, if you can to prevent Dummy from getting into the lead. Your partner leads Queen; you holding A 10 x, and Dummy having K x x. Let the King make on the first round.
If your partner leads a small card up to strength in Dummy’s hand, he is either inviting a force, or trying to establish a long suit. Under such circumstances, if you have the Ace, play it, and lead a second round of the suit immediately, which will settle the question.
If you have Q J 10 of a suit in which partner leads King, play the Jack, so that he will count you for Q or no more, and will not go on with the Ace.
IN GENERAL. Both the adversaries of Dummy should adopt the usual tactics for unblocking, etc., especially in no-trumpers, and in some cases Dummy’s exposed cards will make the matter more simple. For instance: You hold A Q alone, of a suit which partner leads. If you are the pone, and Dummy has not the King, play Ace and return the Queen.