I hardly play in tournaments these days, but lately have been playing in an online game. I used to get material (to write about) from playing--and now the online game has become a fertile source. I love to look for "lesson deals" and remarkably found 5 of them in one short session.
The first 3 are "situations" and then you can play out the final 2 full deals on Vu-Bridge.
1) I opened 1 and ended up in 4 after LHO overcalled 2. The Q was led and dummy had a singleton A and 109852. I had K10x and 76.
What is the lesson?
Why didn't LHO lead a high heart (chosing to lead from something like QJxx or the like instead)? He must not have AKJ or KQJ. He would have led a high heart from either of those holdings. The only possible layout is AQJxx on your left and singleton K on your right (and so it proved to be). It is another issue as to whether or not that might help you in the planning of the play.
2) Defending a heart partial I had AKxxx and a singleton diamond. My partner (who had raised spades) led a low spade. Dummy had xx and KQJx. I won (with the K) and shifted to my singleton. Partner won the A and returned a high diamond (the 10). This should be a suit-preference play. Whenever you are giving your partner a ruff, a low card indicates a preference for the lowest remaining suit and vice versa. So, I underled in spades to try to get another ruff. But, partner had only Jxx. He should not lead a high diamond (suit preference) since he knows I don't have AKQ (I would have won the queen-the cheapest at trick one). So, he should lead a medium or low diamond. He had nothing in clubs, but still, he cannot lead the 10 knowing I will underlead (I did) and declarer will win the Q. If I had won the first trick with the Q, then indeed he should play a high diamond if he has the J.
3) Against 1NT (all pass), I led a 4th-best club from Q10842. Dummy had A3 and played low. Partner won the king and returned the 7. Declarer played the 5 and then the 6. What is going on? Unless partner started with 4 (where his correct return is low), declarer remains with J9. If partner had 9x or Jx remaining, he would have returned a high card. When next you get in, don't lay down the queen, because that gives declarer an undeserved trick. Declarer had J965 and careful card reading would prevent you from making an error. (If partner had 4 clubs, there would be plenty of time later--against only 1NT to continue to run the suit.)