I'm a foreigner, and I had to learn English grammar together with the English language. I still stumble, mostly on articles. And I still study it, to get better.

Grammar is part of your writer's toolbox. 

How can you expect to write well if your tools aren't as sharp as they need to be, and if you don't know how to use them?

Yeah, yeah, yeah. Every teacher told you this, probably since elementary school. And yeah, grammar is hard, and learning how to use it right sucks.

However, when you master...