Why is "יין" pronounced "Yayin"? I thought, based on the hints and tips that it would be pronounced "Yon"; as all it says is that "Yod" is like "Y" in "Yes", and that "Nun" is just pronounced "N". So how do you work out how to pronounce a word? Am I just dumb or have I missed something?
No. The gender and number of the verb are related to the subject of the sentence, not the object. So, if you is masculine, the verb will also be masculine. If you is feminine, the verb will also be feminine. The same with plural.
So, it'll be:
l אתה אוהב יין - you (m) like wine.
l את אוהבת יין - you (f) like wine.
Heru, you have written the sentence correctly in the comment section. The Duolingo computer is programmed to accept the correct answer. It does not make mistakes if you type the answer correctly. There are only two things I can think of that might have caused you to get the answer wrong. 1. You had a multiple choice answer that required you to choose two correct answers, but you only selected one correct answer. 2. You spelled all the words correctly here but made an error when doing the exercise. For instance, I have seen people spell יין with two apostrophizes instead of with two yuds. I'm confident that the next time round you'll get the answer marked correct. Good luck.