It sounds like merele sunta or merele sunte bune
Indeed it sounds as though sunt changes to sunte ...
Merele sunte bune.
Yes,Merele sun bune. Not bun ori buni.
How do you know which form of good to use?
You have to know the gender of măr = apple; it is neuter, and that means to use feminine forms for the plural. So the form to use is bune.