I need to know this stuff to make for clean, concise communication when I copy edit another author's work, as well as for my own use when I am writing my novels and in my nonfiction How-To books. So I'll share my insights with you, distilling down a select pair here to its essence--what it does, how it works in our sentences. If you understand the "why," then it is easier to replicate later.
This post has to do with commas and apostrophes. Before you change screens, answer me this: Do you know how to make "you all" into the proper contraction? It ain't "ya'll" if that was your first response.
Just remember that ONE of the ways to use an apostrophe is to show A MISSING LETTER OR TWO. So, you all would properly become y'all. Do not becomes don't. Cannot (yes, run together, not two separate words--check out Webster's for yourself) becomes can't. Whenever you have a missing letter or two, that is where you place the apostrophe. So "because" would become "she said so just 'cause."
Another usage of apostrophes is to show possession, but that gets involved, what with plural possessive alone, so we'll just skip that until I have the time for another grammar lesson.
There is some debate about the serial comma--for example: She carried her books, lunch and coat in her arms versus She carried her books, lunch, and coat in her arms. I happen to think the first version is the way to go. And for a couple reasons. Number one, it's less messy, less cluttered. But, number two, mainly because of this tip: When using a comma to separate individual adjacent words (apart from the clause-based comma as shown outside of the closing parenthesis here), only use a comma where there is not already an "and" (avoid duplication) or where "and" would still work (ensure clarity). A comma CAN mean "and." You don't need both, in my opinion. Therefore, if you already have an "and" there, don't use the comma.
Plus there are some cases where an adverb modifies an adjective or another adjective modifies an adjective and you don't need a comma because you don't need an "and" there. Here's a sentence to illustrate: They were having pretty cold weather. In this instance, "pretty"--which can be an adjective, noun, adverb or verb--is working as an adverb. "Cold" is acting as an adjective modifying the noun "weather." But you don't even have to have a working knowledge of adverbs vs. adjectives vs. nouns. All you need to know is that you do not need a comma between serial words if "and" would not work there in its place.
Again back to our sample sentence: They were having pretty cold weather. If you believe this sentence needs two commas, insert an "and" where you think the commas should go. Try reading this and maintaining the intent of the original sentence: They were having pretty and cold and weather. Not good.
What if you decide it just needs one comma? Then you end up with a sentence (under the "and" construction) that would read: They were having pretty and cold weather. Still not the meaning intended. Therefore, I contend this sentence needs no commas because "and" does not work in its stead.
Does that help? I hope so. Whether we are authors or bloggers or have need to write something for general consumption, we should communicate clearly. Commas, of course, can have other usages--like setting off an introductory phrase as I already pointed out above or it can set off a phrase within a sentence in such a way that if you took out the opening comma and the closing comma and all the words in between, then the sentence would still make sense when read (like taking out ", of course," near the beginning of this sentence).
The English language is replete with grammar and spelling rules. Some make sense, some don't. But regardless of whether you are writing for a literary audience or a reading-for-fun audience, whether you are writing dialect (y'all) or colloquialisms (soft drink vs. pop) or slang (gonna) or jargon (send the RFA and RFP ASAP to the DA), you now know if you need commas and also where to place the apostrophe when you cut out some letters from the Webster-based spelling of a word.
Even Webster's will give you a short grammar lesson when you look up the word "apostrophe."
Oh, and you should have your favorite reference books from which you can state, "This is why I do this." Be it Webster's 11th Edition and the Chicago Manual of Style 15th/16th Editions (for fiction and nonfiction)--which happen to be my two go-to volumes--or the Associated Press Stylebook (aka AP Stylebook for newspapers), at least have a standard to follow. There are various style manuals out there. Search the web for the medical-related version and others for the one you most need.
While you are searching the internet, Webster's has an online dictionary that now has a tab entitled "New Words & Slang" which may be of help to you. Look for it at http://nws.merriam-webster.com/opendictionary/ or check out http://urbandictionary.com.
A final note. There is such a thing as a "style sheet" within the traditional publishing houses. It is called a "sheet" and not a "manual" because it is just that--pages, not a bound book for sale. For the house where I freelance, they have chosen to override certain Chicago Manual of Style (CMS) rules. The "style sheet" DOES NOT replace the 956-page CMS "style manual." That would be like saying the list of the twenty words that the local third-grade students need to learn to spell by next week will now replace ANY dictionary. No. Of course not.
I hope this sheds a little light on a very complex subject. For us authors, we need to be some order of grammarians to communicate well with our readers. When I was young and devouring books, I sat with a dictionary by my side, looking up words that I did not understand. Now that I am an adult and an author, I really don't want my educated readers pulling out Webster's like a challenge in Scrabble to prove whether or not my word was misspelled. I don't want my novels interrupted by anything--for I want my readers to finish my books in one sitting. To be that engrossed in my creation is my goal. Thus, I plan on removing any such distractions from within my tales.
So turn on spell check and grammar check within your word processing system for your commercial writings, as well as clicking the spell check icon provided for within your blog or your email program or even on the Kindle Boards. Utilizing this simple tool will stop your reader from taking an unplanned detour from your words to consult a dictionary instead.