Grammar rules help to understand the language structure, to see how it normally works.
However, when you learn grammar rules you also have to meet various exceptions that on the one hand may seem confusing at first, but on the other hand they are often the most important cases of the rules. Almost all English grammar rules have some exceptions to them.
One of the ways to remember the numerous exceptions is to make a list of the most common ones. In this reference book you can find the list of fifty most commonly used exceptions in English pronunciation, spelling and grammar.