Let’s regard a set of clauses, or phrases and use of commas.
The dependent restrictive clause. Eh? The what? The clause that follows the main clause and cannot be omitted without changing the meaning of said main clause should not be separated from the main clause by a comma. A whole lot of words to basically say if you leave it off you get a sentence which means something completely different.
An example might help you understand. 🙂
Lucy will agree to promote your book if you accept her way of doing so.
If you leave off “if you accept her way of doing so.” the above sentence has a whole different meaning. There is no restriction whether or not Lucy will promote, while in fact there is. The same goes for the below example. Try it, I know you want to. 🙂
The author requesting a promotion was baffled at how easy he could get her to promote his work.
I’m sure you got it, right? Let’s look at the nonrestrictive dependent clause now. It’s the clause that can be omitted without changing the meaning of the main clause should be set off by a comma.
No, i’m quite serious. Look at the below example and think about it.
The author opted for a one-day promotion, although he would have wanted Lucy to review his book.
But if the both, restrictive and nonrestrictive, precede the main clause they should be set off by a comma.
If you accept her way of promoting, Lucy will agree to advertise your book. Although the author wanted Lucy to review his book, he opted for a one-day promotion.
And then there’s the adverbial phrase at the beginning of a sentence. That one is usually set off by a comma, but not if it is a short introductory one.
Don’t panic!
After reading Lucy’s review, the author wished he’d not insisted but opted for the promotion.
After breakfast he read Lucy’s review.
See? Not that difficult at all, or is it?
If the introductory adverbial phrase immediately precedes the verb it modifies the two should not be separated by a comma.
On the sofa sat an author with Lucy’s review on his lap.
But if that adverbial phrase, or clause is in between the subject and the verb it is usually set off by a comma.
The author, after reading Lucy’s review, sat on the sofa at a loss for words.
But I’m sure you’re not, or are you?
But if you don’t feel that way, do let me know and keep your eye out for more to come on the subject of the dreaded comma. Do let me know if you have questions and I’ll be more than happy to answer them, if I can. 🙂
All That's Written ... 
I think I need a drink!
(“…well, I’m glad I read this and it’s good to learn how something should be done, even if I myself tend to use a comma like someone would take a breath of air to continue speaking I suppose…” ventured the aware that most of his readers were those forum goers freshly off facebook, adding “…just that the ommision of the [i]full stop[/i] stops me from going overboard on length here, in short it’s one [i]run on sentence[/i] per post with me…”, “…ah yes, that delusion of you reliving your youth in writing isn’t it goblin, where perfection would betray that age of yours, and how vain you seem by it too…” intervened the slot but the goblin just gleed at the thought of being [i]as young as one writes[/i] once more)