In this module you can learn what a variables and expressions are and how they are used in algebra.
There are no rules that determine what letter is used as a variable.
Variables always represent some value, even if that value is zero.
The number that comes just after "x" is not "y"; it is "x + 1".
Study and practice
Use these search terms to find your own resources:
what is a variable
algebra variables
working with variables