Your vocabulary looks good, it’s just the grammar that is poor. In school, we generally study grammar from around 3rd to 11th grades, so even with native speakers it takes about 9 years to learn well. The main tools used are sentence diagramming (or sentence skeletons) and grammatical reference books (which will teach you how to diagram sentences according to parts of speech, and help with things like subject-verb agreement.)

READING is fundamental. You will not find school literature with poor grammar. Start with primers and keep going up through the grades of reading levels. Don’t be embarrassed to read children’s books. There is something to be learned at every step, and you need to know the early concepts to understand the more complex ones.

Your best source of good usage of language is with good literature… read as much as you can, and read out loud if it doesn’t disturb anyone. Practice pronunciation as well as reading, and ALWAYS look up words that are unfamiliar. Keep a dictionary close to you, and use it unashamedly. Learn how to read the diacritical markings, so that you can work out pronunciation of unfamiliar words on your own, if you need to.

You could also look for an English teacher… you need someone who is consistent, who can help you with questions you might have, and who is familiar with the tools.

Etiquette training actually exists. This includes proper mannerism including speech. Search the Internet for listings in your area or post an ad in the paper. For immediate correction, begin speaking into a hand-held tape recorder and listen to your voice on playback. Once you hear your dialectal twangs, you will know what to target first. I am from New York. I know first hand about dialect. Keep in mind that no matter how you modify your dialect, if you are susceptible to other people’s mannerism’s, you will need to assess and change that about yourself. My friend changed all this within herself successfully, but it took her about 10 years with conscientous mindful improvements. Ironically, the person that I find most distateful in this entire Universe has a smooth mid-western dialect and she is from New York which, of course, makes me stomach her less.