I spend a lot of time reviewing euchre hands, I typically see novice players giving little thought to cards already played. They have a tendency to worry about following suit and not much more.
In this example, the dealer is sitting in the South seat and turns up the 9 of hearts. The player in first seat, sitting with 3 trump plus an off suit ace orders the dealer to pick the 9.
The maker starts the hand leading the left. North seat follow the trump lead with the 10. Their partner(E) shows void in trump so they throw off a useless 9 of clubs. South holds the right and uses it to take the trick. The maker loses the first trick and there are three trump out of the picture.
Having denied the maker a 2point hand, South now leads back the 9 of trump. Occasionally leading back a trump will result in euchre. It may also serve to confuse the maker. No matter what, they only can make 1 point on this hand. West plays the queen. North seat overtakes the queen with his king. East can do nothing to help. There are now 5 trump gone and the N/S team has their second trick.
North plays the ace of clubs. They only need one more trick for a euchre. The maker now finds himself in a situation in which he has no control. He has to follow suit with the queen.
Let us take a closer look at where the maker went wrong. One the 1st lead he was correct in leading the left. This would either be good or pull out the right. Things quickly go downhill from there. Once south led the trump 9, had the maker done a review of cards he would have known his ace was the most powerful trump left and there was still a trump out.
By playing the ace of trump and then leading that off suit ace, the maker would have been on his way to collecting a point.