When to Use a Comma
In a series
Use a comma to separate the items in a series of three or more words, phrases or clauses.
Using a comma before the conjunction that joins the last two items in a series to prevent confusion regarding whether the last two items in a series are related.
In the previous example “advertising and public relations” a comma can be not used if the word “relations” refers to “advertising”. If “advertising” has nothing to do with “relations”, a comma should be used to avoid misunderstanding.
To separate independent clauses in a sentence
Use a comma to separate independent clauses that are joined by the coordinating conjunctions “and”, “but”, “yet”, “for”, “nor”, and “or.”
To separate parts of a sentence
To separate a subordinate clause or long introductory phrase at the start of a sentence from the main clause
After a dependent adverbial clause or prepositional phrase that starts a sentence
To separate an introductory adverb from the rest of the sentence
With nonrestrictive phrases
Use a comma to set off non-defining relative clauses.
With parenthetic text
Use commas to set off short parenthetic material.
Use commas to set off components of an address when the address appears in a sentence or on one line.
Use commas to separate the components of a date.
With “for example” and similar expressions
Use commas to set off expressions such as “for example,” “that is,” and “namely.”
The comma should be used between the person's name and the greeting. The reason is “direct address”. We use commas to show that we are talking to the reader, not about the reader.
Exception: Do not use a comma with the greeting Dear, as in “Dear Claudio:”, “Dear Claudio,” or “Hi Claudio,”.
The “Dear Claudio” greeting is shown with both a colon and a comma. The colon is used in formal business emails. The comma is used in less formal ones.
When Not to Use a Comma
When several adjectives are used as a single modifier
This applies to adjectives that describe different qualities of an object, like its color, size, weight, etc.
Between two short independent clauses
In normal order of a sentence
If a dependent adverbial clause or prepositional phrase appears in its normal order in the sentence. This usually concerns adverbial clauses at the beginning of a sentence.