IV. Punctuate your sentence while you are writing it.
V. Understand what you are printing. This is of supreme importance. Punctuation is an aid to understanding. You cannot correctly punctuate anything that you do not understand.
THE COMMA
The comma is by far the most difficult of all the punctuation marks to use correctly. Usage varies greatly from time to time and among equally good writers and printers at the same time. Certain general rules may be stated and should be learned. Many cases, however, will arise in which the rules will be differently interpreted and differently applied by different people.
The comma is the least degree of separation possible of indication in print. Its business is to define the particles and minor clauses of a sentence. A progressive tendency may be seen in the printing of English for centuries toward the elimination of commas, and the substitution of the comma for the semicolon and of the semicolon for the colon. Compare a page of the King James version of the Bible, especially in one of its earlier printings, with a page of serious discourse of to-day and the effects of the tendency will be easily seen. It is part of the general tendency toward greater simplicity of expression which has developed the clear and simple English of the best contemporary writers out of the involved and ornate style of the period of Queen Elizabeth. An ornate and involved style needs a good deal of punctuation to make it intelligible, while a simple and direct style needs but very little help.
This progressive change in the need for punctuation and in the attitude of writers toward it accounts for the difference in usage and for the difficulty in fixing rules to cover all cases. The present attitude toward punctuation, especially the use of the comma, is one of aversion. The writer is always held to justification of the presence of a comma rather than of its absence. Nevertheless it is quite possible to go too far in the omission of commas in ordinary writing. It is quite possible to construct sentences in such a way as to avoid their use. The result is a harsh and awkward style, unwarranted by any necessity. Ordinary writing needs some use of commas to indicate the sense and to prevent ambiguity.
Always remember that the real business of the comma is just that of helping the meaning of the words and of preventing ambiguity by showing clearly the separation and connection of words and phrases. If there is possibility of misunderstanding without a comma, put one in. If the words tell their story beyond possibility of misunderstanding without a comma, there is no reason for its use. This rule will serve as a fairly dependable guide in the absence of any well recognized rule for a particular case, or where doubt exists as to the application of a rule.
Reversed, and usually in pairs, commas mark the beginning of a quotation.