Let's make a distinction between grammar, spelling, and puncuation. A native speaker understands that some syntatical structures sound wrong and is able to correct them to sound naturally. Most English as a second language students understand grammatical rules but they often make awkward sentences that just sound weird to native speakers. I heavily belong to this group like many ESL learners - and even their teachers.
Spelling and punctuation are different from this. You just have to take a book and start looking things up about punctuation: you look up
however and you can figure out of the sentences which usages need or doesn't need a comma. You go to the appendices and learn the usage of commas, periods, semicolons, and hypens without being able to form a correct sentence in the Present Continuous (
I going to school every day, however, I don't like doing it). I'm using this example because I know that many don't use commas in this sense of
however. My point is that you may not know how a correct English sentence looks like, but you may know where to put commas. This is true vice versa. You may write the same sentence correctly without using commas before and after
however, which is incorrect punctuation, but the sentence suddenly sounds right.
Nowadays many schools refuse to mark students down for bad spelling or grammar on the grounds it'll hurt their self-esteem. 
That's a mistake because English spelling and pronunciation differ very much.