Here's your hand, sitting south in third seat:
Partner opens 1H in first seat. Regardless of what system you play, Standard American or 2/1, you can only respond 1N here, even though you have a six card suit and a singleton. You have 7 HCP, so you must bid. You're too weak to bid your deplorably bad diamond suit.
Partner reverses with 2S (which promises at least 17 HCP and 5-4 distribution, or 6-5 with opening hand values). You have to bid again. But the value of a reverse is that it promises at least 17 HCP, so some of the values you know for bids are now less. For instance, normally a second bid by responder of 2N promises 11-12 HCP. But since you know partner has at least 17 HCP instead of a maximum of 14 HCP with a normal response, your bid of 2N will now only need 8-9 HCP. Even though you only have 7 HCP, you do have a six card suit. And, anyway, you don't have any other response. You can't raise spades since partner probably only has 4 and you clearly can't raise hearts with a singleton. Really your only bid with this hand is 2N. If your diamonds were better, like AQxxxx, then you could bid 3D, which would generally be a drop dead bid that partner should pass.
But you do have clubs stopped, and your diamond suit, weak as it is, is long enough that it is stopped enough so that opponents shouldn't take more than three tricks in it, if that. So your only bid is 2N, even though you only have seven HCP. Partner with a huge hand containing 19 HCP bids 3N. Here's the four hand layout:
♠ 852 ♠ Q973
♥ AJ92 ♥ 863
♦ JT ♦ KQ5
♣ J862 ♣ 754
West North East South
1H P 1N
P 2S P 2N
P 3N All Pass
Opening Lead 2C.
Before I comment on declarer play, I want to say that even though it's standard to lead fourth from your longest suit as an opening lead against no-trump, leading low from a jack high four card suit is generally a destructive lead because the Jack will stop a suit and if you lead from it declarer often wins the trick with the 10. If the dummy in this hand had KQx instead of KQ tight, it's a perfect example of how a lead from Jxxx gives declarer a trick she wouldn't get otherwise. Playing low from dummy, she takes the trick with her Ten in her hand. She'd never get that trick if the lead from Jxxx isn't made.
As to declarer play, you count your tricks. You've got three club tricks (because the King Queen is tight on the board, you can't take the opening lead with your 10, so West's Jack will end up taking a trick if you try to take all three club tricks), one diamond trick, three spade tricks, and one or two heart tricks. But as the hearts actually lie, you're only going to take one heart trick because if you lead your singleton and West plays low, you have to lead the hearts from the board and you're doomed to lose to the ace and the Jack, at least.
So your only hope to make three no-trump on this hand is to hope for a 3-2 split in diamonds. It doesn't matter where the honors are. What is important, however, is that you keep two entries to your hand, because you have to lead diamonds three times to set up three diamond tricks, which means you have to get to your hand twice, and you only have two entries.
So you take the opening lead on the board with the Queen of clubs, lead the ace of diamonds and a low diamond, ridding the board of diamonds, which is won by East's Queen. Now there is only one diamond out. East returns a club to the singleton King on the board. But you must take this in your hand with the ace, even though you have to drop the singleton King on the board under it, because if you play low so that the king wins on the board, you only have one entry to your hand, the king of spades. Here's the layout at the point East leads his second club:
♠ 852 ♠ Q973
♥ AJ92 ♥ 863
♦ ♦ K
♣ J86 ♣ 75
If you use the king of spades to get to your hand to play another diamond to get the last diamond out, you can't get back to your hand to play the three good diamonds that you set up because you don't have another entry.
So you have to play the ace and drop the king under it. You lead your third diamond. East takes it and returns a club. West wins the jack and returns a spade, which you win in your hand with the king over East's 9. You've made your contract; four diamonds, three clubs (your 9 held up for the last club trick), and two spades without ever touching dummy's beautiful hearts.