If someone wasn't a big reader, wasn't taught grammar in school and started reading, say, Stephen King at age 14, didn't really deviate much and picked up construction and writing from that, that would be an issue.
Same as anything else. If someone has a good piano teacher who grounds him in the basic skills, and that student grows into a competent player, he can then try different styles and methods with no problem. He knows the 'correct' way to play and thus can hear the difference when another player is playing with half the indicated pauses in a section for particular effect or with more emotion or what have you.
By the way, there are a lot of sites with grammar instruction and games to play to check your understanding - they're for kids but that's where you missed the instruction so that's what'd be good to start with, I think.
Try googling middle school grammar games or practice and you'll see lots of sites with different types of practice and training things - you can use those with the books to see how much you're advancing and etc.