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10. Save your ace to capture dummy's king. The ace gives up a trick if declarer has, say, Qxxx or similar.  

10. When you can't beat dummy (or partner), your card should be a signal. In this case, it's attitude and you like this suit with the king which you show by playing your higher spot card. 

3. Playing 4th best leads, you can use a tool called the Rule of 11 to determine how many cards higher than partner's lead are in the other three hands. 11-6 (partner's lead) = 5. We can see all five cards higher which means that partner led the 6 from Q1076. If you play low, partner can stay on lead. Declarer made an error in not playing the 9; don't help out by playing the 8. 

9. Playing third to the trick, you play the lowest of touching honors. If declarer has the queen, he's always entitled to one trick. If you play the ace (or jack), declarer can take two tricks. If declarer doesn't have the queen, then declarer will not win a trick unless you give it to him. 

K. Third hand high. In a suit contract, partner won't lead low from an ace, but in notrump partner could have Axxx or Axxxx. 

K. We normally don't signal with a trick, but we are afraid of accidentally blocking this heart suit if partner has, say, Jxxxx. Playing the Q would deny the king.