Commas should be used to separate independent clauses that are joined by coordinating conjunctions, including the conjunctions "and," "but," "or," "for," "yet," "so" and "nor." Commas are also used to separate three or more words, clauses or phrases in a series; however, some style guides do not use the serial comma for a concluding item in a list.
Commas are also used after introductory phrases, clauses and words that come before a main cause. Some starter words that indicate an introductory clause include "while," "when," "since," "if," and "because." The comma is not necessary when the main clause is followed by a dependent or subordinate clause. Long prepositional phrases of four words or more, nonessential appositive phrases, absolute phrases and participial and infinitive phrases should be followed by a comma.
Some common introductory words must be followed by a comma. Examples include "however," "yes" and "well."
Commas are also used to set off phrases, words and clauses that are not essential the sentence's meaning. In this case, one comma is used before the phrase, word or clause and one after it to indicate its end.
Commas are used when more than one adjective is used to modify a single noun. These adjectives are called coordinate adjectives, since they both describe the noun in question. A good rule of thumb in deciding if the adjectives require a comma to separate them is to determine if the sentence would still make sense if the word "and" were to be inserted between them. If not, a comma is not needed.