[akwoo]

I should've gotten this right but I got this wrong.

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You hold the North cards. 2♣ was forcing to game; 2♠ did not show extra values but did imply real diamonds. (East-West were actually playing Precision, but I don't think it matters here. Also, it's matchpoints, but it doesn't matter - the relevant trick is the setting trick.)

I assume you lead the ♥K. Dummy takes the A, partner follows with the 3 (standard carding, presumably attitude), and declarer the 2.

Declarer cashes ♣AK and ♦Q, all following, then exits to you with a club, on which your partner discards a heart. Presumably you lead the ♠4, to partner's A. Partner returns a heart, declarer plays the 9, and you take the 10. What do you do now? (You have ♠Q96 ♥Q5 ♦J and dummy has ♠T85 ♦AK8)

Spoiler

Let's count carefully. Everyone has 6 cards left. Declarer must have 3 clubs left, as well as the ♠K and the ♥J, and one more card.

Okay - I didn't tell you some crucial information. Hopefully you noted this and remembered it! What would you do if partner had discarded the 6 and led the 7 (in hearts)? What if partner had discarded the 6 and led the 8? Discarded the 8 and led the 7? Discarded the 7 and led the 8? Have you and partner discussed this? Presumably some of these tell you the information you need, and the rest of the possibilities are impossible and indicate partner wasn't paying attention?

Spoiler

If declarer's last card is a heart (which must be higher than your 5), you must lead a spade. If declarer's last card is the ♠J or a diamond, you must cash your hearts. Did your partner tell you whether they started with 3 or 4 hearts? Did you pay attention?

Spoiler

As it happened, partner played the 6 and the 7, but I didn't put it all together and cashed a heart.

[pescetom]

Interesting, thanks. I agree partly with what you said in another thread about the difficulty of a relatively simple problem like this for the majority of intermediates. But I'm not sure this is evidence of only a few candidates being really capable of learning. In my experience it is much easier to learn bidding and declarer play in trumps than declarer play without trumps and defence play in particular. The last mentioned is something you learn mainly from years of experience and only if you manage to attract partners at least as good as yourself. I'm still working on it myself, not close to 10,000 hours yet.

Probably demonstrating just that, I think that what I would do now at the table (having read and trusted odd count or even not) is to throw in the dummy with diamonds