For some reason, I can't explain why all the grammar and stuff needs to be there, however, I know it all (or most of it).
For the apple example, it isn't exactly an accurate explanation on the sounds. "Tommy found an orange in his backyard." works. Even though the beginning sound of "orange" is different from "a", the "n" is needed. This is how my teacher used to explain it.
"Vowels don't like each other. In fact, they don't like each other at all. That's why we place an 'n' in between them to keep them separate."
Thus, whenever you have 'a' and something that starts with a vowel in the front, you must put a 'n' in front of it. (Man, I was tempted to write "an 'n'".