NO ONE VULNERABLE
10 7 2
Q J 8 7 6
A K 10
10 9 3
A 6 4
8 7 6 4 3
West leads the five of spades and dummy wins with the queen. You should play the ten on this. The ten obviously denies the ace or king since you would have covered the queen if you could.
If you had the jack and ten, you would play the jack. When you play the ten, it tells partner you do not have the jack, but you do have the nine. This will be useful to partner when he gets in. Your ten is NOT suit preference. It is just telling partner what you have in spades. At trick two, declarer finesses the queen of diamonds to West's king. West leads the heart three and you take your ace. What now? Why?
Return the six of hearts. West led a little heart, which shows interest in hearts. If West wanted you to return a spade, he would have led a high heart spot to tell you he was just trying to get you in. It is important to have the understanding that when you shift to a little card you are showing intertest in the new suit and when you shift to a big spot card, you do not like the new suit. How else can you tell partner what you want him to do? NOTE that you should return the six of hearts. If you had started with the A654 of hearts, you would return the four. You are trying to tell partner how many hearts you have. If you started with four or more cards in the suit partner leads, you return your original fourth best. With three only, you return the higher card. Hopefully, partner can read it.
If East-West don't have this agreement, East might return a spade, allowing declarer to take his ten tricks.
This is a frequently recurring theme. You make your opening lead and at the end of trick one, you usually know whether it was a good lead or a bad one. Later, when you get in, you will need to put partner in to make a return for you. Remember the above rule. If you want him to return your new suit, lead a little card. If you want him to return your original suit, lead a high card. It would be a shame to have partner return the wrong suit.