I often claim that the best way to learn about the mechanics of English (grammar, spelling, punctuation, etc.) is through exposure. Surround yourself with words. If every time you ever see the written word, it is flawless, you will only understand it in its flawless form (it scares me how much more difficult that becomes in the digital age).
The problem with sheer exposure is that you will learn the mechanics, but you won't understand them. You may know when a comma feels natural or a semicolon fits, but you don't know why. Fortunately, a minimal amount of studying will be enough to figure it all out.
If you have been immersed enough in standard written English that you know when to use the right mechanics, then you are already good at noticing patterns. If you learn about English structures like dependent and independent clauses, prepositional phrases, parts of speech (noun, verb, adjective, etc.), then you will be able to see more of the patterns.
In fact, with exposure to correct mechanics and a basic knowledge of the language, you can literally derive the usage rule for any mechanic just by seeing how it's used and explaining the pattern with the proper terminology.
I really do recommend actually learning the rules for English grammar. Just because you can get by doesn't mean you're going to get it right all the time. When I was very young, I remember solving the "then/than" issue by saying the sentence out loud and listening to if the word sounded more like an 'a' or an 'e'. I'm sure I got it wrong a lot. Once I actually learned what the rule was, I never made that mistake again.
I will say that you will feel like a genius and a badass if somebody ever asks you about some rule of English and you can write a sentence with an example of it, stare at it, and then solve it and explain it to them. It's awesome realizing that you know even more than you realized you did.