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The ACT tests over most comma rules, but not all of them.  I didn't want to post a list of ACT-only comma rules and have anyone think it was a complete list.  Consequently, I have this complete list below and the tested rules list on another page.

1.  Use commas to separate items in a series.  The items can single words or phrases or clauses—anything. The exception is if all items are separated with a conjunction, no commas are used.

    I like fruit, vegetables, and ice cream.  I like fruit and vegetables and ice cream.
    He tried working for his mom, loafing for his living, and singing for his supper.

2.  Use a comma to separate two adjectives that precede a noun under the following circumstances: if the adjectives can be transposed (turned around) or if the word and can be placed between them without affecting the meaning. Participles without phrases, since they act like adjectives, are also targeted by this rule.

    Mr. Roden is a wonderful, exuberant man.
(Use a comma since the adjectives can be transposed--Mr. Roden is an exuberant, wonderful man—and and can be inserted— Mr. Roden is a wonderful and exuberant man)

3. Use a comma to separate independent clauses in compound sentences when the clauses are joined by a coordinating conjunction (unless they're very short clauses of two or three words).  When they are joined with for or yet, use a comma regardless of their length.

    My leg hurts badly, and I have a doctor's excuse.  I loved and I suffered.  I loved, yet I suffered.

4.    Use a comma after introductory elements.  Introductory elements precede any independent clause.  Consequently, they are often found at the beginning of sentences, but look for them elsewhere in compound or compound- complex sentences.
4a.  Mild interjections are interjections not strong enough to require an exclamation mark. These include words like why, no, oh, well, and  yes --unless they're part of the sentence as in the second set of examples.
        Why, I guess I will.  No, I won’t go with you.
        Why won't he go?  No one will go.

4b.  Prepositional phrases of four or more words (unless the phrase is immediately followed by a verb.
        In the early morning, my knees are stiff.
        After a brilliant sunset came a wonderful night sky.

4c.  Participial and infinitive phrases  (when the infinitive phrase isn’t used as the subject)

        Landing on the moon, the capsule made a dust cloud.
        To get an A, she studied and studied and studied.  To get an A is my goal.

    4d.  Adverb clauses   

        When I go to bed, I check my alarm.
5.  Use a comma to separate the elements in dates and addresses.
    Today is October 15, 1999, in the big city.
    He lives at 1002 Easy Street, Corpus Christi, Texas, in a little house.
    Lebanon, Missouri, is a fine place.  (The comma after Missouri looks odd, but it’s correct. Really!)

6.  Use a comma after the salutation of a friendly, informal letter and after the closing of all letters.
    Dear John,

7.  Use commas to set off nouns of direct address.

    Goober, where is Gomer?
    I wish you hadn't done that, son.
    The reason, my friend, is obvious.

8.  Use commas to set off parenthetical expressions.  Most transitions are parenthetical  and some subject and verb combinations are, too.  (Too is parenthetical when it means also.)

    A booger, I believe, will dry up and blow away.
    The weather, consequently, will be unpredictable.

Essential elements limit the possibilities for the noun they rename or describe by telling which one or ones. The rest are nonessential and are set off by commas. 

9.  Use commas to set off nonessential appositives.

The teacher Mr. Roden is wonderful.   (Teacher doesn’t limit the possibilities as much as Mr. Roden.  Consequently, the appositive Mr. Roden is essential and has no comma.)
 Mr. Roden, a man of wonderful ways, taught us well.   (The appositive phrase, a man of wonderful ways, does not limit the possibilities or tell which one, is nonessential, and requires commas.)

10.  Use commas to set off nonessential  subordinate clauses or  participial phrases.  Clauses that are nonessential begin with which while essential ones begin with that.

      Gilligan’s island, which was a tropical one, had coconuts.  The island that was tropical had coconuts.
      Our basketball coach, huffing like a freight train, finished the race. The coach huffing like a train   
      finished the race.

11.  When dealing with dialogue, use commas to separate the attribution (who said it) from the quote. Note when the comma is inside the quote and when it isn’t.

    “I will do well on the test,” said the student. 
    “I certainly hope so,” the teacher replied, “but studying is the key.”

12.    Use a comma before/after a contrasting expression.  Words like unlike or not signal an upcoming contrasting expression.
I like apple pie, not blueberry.
My mother, unlike your mother, makes green beans with bacon and onions.
13.    Use a comma after single-word modifiers.  These are adjectives or adverbs that are out of their normal position near the word they modify.

Later, I found her by the pool.  (Instead of I later found her or I found her later.)
The pool, cool and inviting, had leaves in it.  (Instead of The cool, inviting pool had leaves in it.)

14.    To prevent confusion between numbers, names, and repeated words and to prevent misreading.

In room 17, 25 people slept.  After Randy, Kyle is my favorite. After defrosting, frozen foods taste good