I know a rule:
I found a pen. The pen was broken.
When I introduce a pen, for the first time, it takes the indefinite article and once, it's introduced, I can proceed with the definite one.
Well, but while writing an essay on, say, on a student (any student), which article would go throughout the essay?
The two main entities of any classroom teaching are a student and a teacher. While a/the student serves as an audience, a/the teacher is a performer. Whenever a/the student asks a/the teacher on any subject, they are bound to answer.
Though I have introduced a student and a teacher, I don't want to get specific. Throughout the paragraph, I want to talk about any student, any teacher, randomly and not specific. Even further, what about the entire essay? Wherever I mention student/teacher which article should I put? Mind it, I don't want to be specific, each time I want to talk on random/any student/teacher.
Maybe, the last paragraph would be...
A/the student's performance depends on their own capabilities, learning curves, and top of all a/the teacher's support.
Now, that is the last paragraph...would 'a' work? I don't want the. Would it be incorrect?