Deal 2
 
  4♠
♠ K 10 7 4
10 4
9 5 3
♣ A Q J 3
 
 WEST  NORTH  EAST  SOUTH
1♠
pass3♠pass4♠
passpasspass

The bidding has been as shown.
Partner led the Q, dummy played the 4 and it is your play.

When you have made your choice click NEXT .

Q

♠ 2
A 8 7 6 2
K 8 4
♣ 10 8 5 2
 


 

 
Deal 2
 
  4♠
♠ K 10 7 4
10 4
9 5 3
♣ A Q J 3
  Play the A.
You dare not play low for fear declarer might win a singleton K.

And when you win the first trick, (the K does NOT fall), you have to decide what to do next. Do not just return the suit like a robot since it is obvious that you have no more winners and you surely won't win any ♣ tricks.

You need to switch to a , and the 4, (low from an honor) is recommended.

Played this way you will defeat the contract by one trick.
If you allow declarer to win the first trick he will pull trumps and discard two red losers on dummy's ♣ and make an overtrick.

 Deal 3 
♠ 9 5
Q J 9 5
A J 6 2
♣ 9 7 4
♠ 2
A 8 7 6 2
K 8 4
♣ 10 8 5 2
  ♠ A Q J 8 6 3
K 3
Q 10 7
♣ K 6