I've started to write some story and at the beginning of that I'm faced with an issue. I'm not clear understanding what the article (the definite article or indefinite one) should be used when my oppopent doesn't know about some object but I do and espesially that object is unique. For example: "When I went to school I used to come to class when the teacher always was there." By "the teacher" I mean my "form master" (or "homeroom teacher", I actually don't know which term is more proper).
So, should I use "a teacher" just because some object I'm talking about is unknown for my opponent or it would be better to use "the teacher" and it's quite clear for opponents that I mean "homeroom teacher" or it would be more naturally to use "my teacher" ?