Right, forgot about A's and B's -- we don't use that grading system over here (although, FWIW, I've seen guidebooks that recommend As, Bs and 1s and 2s, because ABCs is also fine and the 1800s is a thing that has to be distinguished from the 1800s'). There's a lot of regional variation, too, which is inevitable considering that English is used from Brisbane to London and from Alaska to California.
But, seriously, every non-constructed language on Earth has exceptions that you have to memorize along with the rules. English doesn't have that many, and it doesn't have too many rules, either. I'm fluent in two other languages (French and my own), and I'm somewhat familiar with a few more languages of Latin, Slavic and Celtic descent. English is a breeze compared to any of them.
But, seriously, every non-constructed language on Earth has exceptions that you have to memorize along with the rules. English doesn't have that many, and it doesn't have too many rules, either. I'm fluent in two other languages (French and my own), and I'm somewhat familiar with a few more languages of Latin, Slavic and Celtic descent. English is a breeze compared to any of them.