Student or Learner
1) Should I say "understanding in or about"?
2) Should I use "in or at the beginning" ?
I had no understanding in / about chemistry in / at the beginning.
I had no understanding of chemistry in/at the beginning.
Either in or at would work, but generally you would say what it is the beginning of. Then it might make a difference, as in these sentences:
I had no understanding of chemistry at the beginning of the course.
I had no understanding of chemistry in the early stages of my medical training.
The differences are subtle, though; I wouldn't worry about them too much.