I searched but could not find a course of study that teaches "why". Grammar or the rules thereof only tells us the rules, rarely does it explain why it is this way. English is a living language it is affected by the technology, society and the people who use it. The grammar used and the words used are usually through accident, not through a reasoned thought process prior to the usage. As you may know spelling has a lot of rules, but each rule has its exceptions. Like the i before e rule (i before e except after c) then suddenly we are hit with either.... The differences between UK English and American English, such as the use or non use of the word the in such phrases as "the hospital" (US) compared to just "hospital" (UK) is an effect of the society that is using English. Spelling as in in center/centre is again due to the society. Writing professional as in a novelist would require an examination of all novels, especially those "best sellers" to see what it is that makes each writer a seller. There are few hard rules when it comes to writing - a "style" is the usage of the words, clips and phrases as well as the situations and story line. The "voice" of the writer usually has much more to do with who the writer is, where s/he grew up and what influences they are brining to bear on the story or subject. Any writer out there that has made a name for themselves (King, Clark, Jewette, etc) started off by reading, and reading and reading. And then they started writing, and writing and writing. Writing is a skill that is learned best by doing, not by having somebody tell you why the language works, but by using the language and honing ones art on the rocky shoals of publishers and critics time and time again, in many cases not selling a single sheet of paper for many years.

I think that you should read a short history of the English language. Learning about the struggles and triumphs that the language has faced will inform you greatly about the Anglophone mentality. A history will provide a context for the logic of the language.

I will give you this. If this is your second language, you certainly do write very well. I wish I could communicate in another language this fluently. Good luck to you.