*Not a teacher
As I have written above, I am not a teacher, but the definite article is a quite heavy issue. I don't think that somebody can explain all about it in a forum post. There are a lot of rules, exceptions and disputes about its usage. You had better consult a grammar book which treat this subject in depth and, if you have a specific question, come back here and ask, and I am sure that somebody will help you out.
Student or Learner