I want to normalize this on 1NF, 2NF and 3NF. I know a little bit how to normalize tables in practice but I don't know where to start using letters. Thanks
R (A, B, C, D, E, F, G, H, I, J)
AB -> C
A -> DE
B -> F
F -> GH
D -> IJ
L: A, B
M: D, F
R: C, E, G, H, I, J
AB*
ABC
ABCDE
ABCDEF
ABCDEFGH
ABCDEFGHIJ
How to find the primary key here?
You always start checking with the lowest normal form and move upwards. If you are not given what these letters stand for then you have to assume that it is in 1NF.
To check 2NF:
The key here is AB. 2NF is violated when given a functional dependency X->Y if right-hand side is non-prime and is not part of X and left-hand side is a proper subset of a key (which here is AB). So here you can directly see one of the functional dependencies A->DE violating this and hence it is not in 2NF. Now move on to check for 3NF. As it appears to be a homework question, I will be willing to help you any further only once you show some progress on that.
I will suggest you to watch these videos. (especially 28a and 29a). This should get you started!