Commas Indicated by Key Words

 A comma must be used with coordinating conjunctions between two independent clauses.

• You put up a good fight, but Nancy put up some really groovy wallpaper.
• We dine at five, for we have a ball to attend this evening.
• The television is on the fritz, so we are going to whack it with a baseball bat. 

TIP: In order for the comma to be needed with the conjunction, it must be joining two independent clauses. In other words, there must be a subject + verb before and after the conjunction for a comma to be needed.

A comma follows the exclamatory "Oh" but not the direct address "O."

• Oh, how ludicrous that sounds!
• O gentle king, have pity on a poor charwoman's soul! 

Parenthetical expressions starting with not should be set off by commas.

• The thistles in my pie, not the strawberries, are making my tongue hurt.
• I came to you, not for a lecture, but for your help.

When a series concludes with etc., the etc. is set off by commas.

• At the Salad-o-Rama, we filled our shopping basket with lettuce, tomatoes, carrots,etc., but no croutons. 

A comma is used to set off terms such as that is, namely, i.e., and e.g. when they are used to introduce a series or an example.

• Some students, namely, Morticia, Gilligan, and Dano, had better practice their brainstorming skills. 
• I suggest you leave right now,i.e., beat it, if you know what's good for you.

A comma is used to set off conjunctive adverbs. 

• Your application has been denied; however, we will keep it in mind for future consideration. 

TIP: Don't forget that when you join two independent clauses with conjuctive adverbs, you must seperate the clauses with a semicolon, then follow the adverb with a comma!