"John has a nice watch."
Translation:Jánosnak van egy szép órája.
It is a matter of emphasis most of the time, but there are of course some rules, too. Sometimes the difference is small, other times it is more significant.
Here, you can think of it as an answer to a certain type of question.
yes or no, existence: Does János have a nice watch? Yes, he does:
Jánosnak van egy szép órája.
"multiple choice": What does János have? He has a nice watch:
Jánosnak egy szép órája van.
If you look closely, you see that what is emphasized is closer to the front of the sentence.
But how can I know which emphasis is correct here since I don't have any context? It marks one of those two as being wrong, but I don't see how I can guess which is the intended emphasis based on the provided English sentence. Most of the time I just have to guess and hope for the best.