Before reading this post make sure you know what a finesse is. Click here if you don't.
Try playing along at home. Deal out the cards and play it out yourself! That is the best way to learn.
I need to take 3 tricks in this suit. How do I do it?
Dummy:
♠AJ2
Declarer:
♠K43
Hmmm. I have 2 tricks for sure. The A♠ and the K♠. If only that J♠ was the Q♠! But it isn't and I need to score a trick with the J♠. That's my only shot for taking 3 tricks.
I have a couple of chances here. I can play the A♠ and the K♠ and hope that the Q♠ drops underneath one of them. This would mean that one of the opponents only had 2 ♠ and one of them was the Q♠. Not very likely. My other chance is to finesse the J♠. Yes, finesses are not only for Kings and Queens. Other cards can get finessed also.
In order to figure out what way to finesse I now have to think a little bit. Who do I want to have the Q♠? Does it matter? How will I play the suit?
So let's think about this. If I give the Queen to my RHO I can play the J♠ and then my RHO will have to play their Q♠. Then I can win with my K♠. I've found the Queen!
While this is true that I have found the Q♠, it is also true that this did not help me at all. I now have this left:
Dummy:
♠A2
Declarer:
♠43
There is no way my 4♠ will ever be good. This did not work at all.
Okay, let's switch that to my LHO having the Q♠. Now I can play my J♠ from Dummy and play low.
Oh wait. That doesn't work either. My LHO just wins the Q♠.
Try dealing out the cards and figuring this out yourself. Give one opponent three cards and the other opponent four cards. Either one can have the Queen. Then play the suit out so that you get three tricks. If it doesn't work giving the Queen to one opponent, give it to the other opponent. If at any point you play your J♠ and the next player to play has the Q♠ they will cover your J♠ with their Q♠.
So in the end you need to tell me which opponent you gave the Queen to (RHO or LHO) and how you played the suit. That means which card did you play first and then what did you do.