Can you help me with this Spanish grammar rule?

I’ve been taking Spanish for a while now, but my grammar still needs some work. One rule that I’m having problems with is under what circumstances is it necessary for the “y” in a phrase to be changed to an “e” and when the “o” should be changed to a “u”. My teachers have corrected these mistakes in my essays, but never explained why it’s wrong.
So, what I’m meaning is when I say: “me gustaba leer y quería aprender más de todos los temas en general”, why is “y” acceptable, but not “e”? Does it mean the same thing? When should I use it, and why?
I’m asking the same thing about using “o”. If I write, “no me gustaba asistir a fiestas o charlar con los niños”, why does “o” work, but not “u”, and what does that mean? When should I use it, and why?

If you can give me examples, that would be greatly appreciated.