Since Esperanto is partially based in German, it adopts that pronunciation of the letter "j," at least as far as I've seen, which is to pronounce it as you would a "y." For instance, in German "ja" is pronounced "ya," "jaeger" is "yaeger," and "Junker" is "younker." Hope that helps!
Here's some simple rules to help get it right, and it all depends on the verbs used:
If you're saying someone is something e.g. "Ni estas belaj leteroj" (We are beautiful letters) you use only the -j ending because we're stating we are beautiful letters. Imagine estas means equals. So nothing is happening to the belaj leteroj. It's just that we are beautiful letters.
If something is happening to something, you use the -jn ending because the -n ending makes it an accusative case. What does that mean? It means something is happening to them. This can be anything e.g. havas (have), amas (love), or skribas (write). So if you say "I have beautiful letters you need to say "Mi havas belajn leterojn" with the -jn ending because something is happening to them.
You probably already know, but if something is plural it needs the -j ending.
If something is happening to something it needs the -n ending.
I hope that helps and that it makes sense. as well.