Partner then plays the 8 and declarer the 10 as shown below. And you?
Choose One:
2
J
Other
Answer: 1 -
2. From trick one (and the bidding), you know declarer has the Q (partner would have played the Q with KQ). Now you know declarer started with Q10xx.
How?
Because with 2 cards remaining in a suit, we return the higher. With 3 cards remaining we return lowest.
Partner's 8 can't be from 3 remaining, so is from 2 remaining (it won't be his only one; declarer would have responded in a 5-card heart suit).
Knowing the heart layout, you can tell that winning your J will limit your side to only 3 heart tricks. Better is to duck and hope partner has an entry (likely). These are the remaining cards:
After declarer wins the 10, he has to eventually play diamonds. Partner will win and play another heart and the contract is set.