To begin with:
in the end
In the end suggests that something happens after a lot of changes, problems or uncertainty.
We made eight different plans for our holiday, but in the end we went to Brighton again.
I left in the middle of the film. Did they get married in the end?
The tax man will get you in the end.
at the end
At the end simply refers to position at the end of something. There is no sense of waiting or delay.
A declarative sentence usually has a capital letter at the beginning and a full stop at the end.
I wish I was paid at the beginning of the week and not at the end.
on the end
Comment on the end of the book.
He made a speech on the end of the world.
Student or Learner