Comma (,)

Use a comma after the first independent clause when you link two independent clauses with one of the following coordinating conjunctions: and, but, for, or, nor, so, yet. For example:

I am going home, and I intend to stay there.

It rained heavily during the afternoon, but we managed to have our picnic anyway.

They couldn't make it to the summit and back before dark, so they decided to camp for the night.

Semicolon (;)

Use a semicolon when you link two independent clauses with no connecting words. For example:

I am going home; I intend to stay there.

It rained heavily during the afternoon; we managed to have our picnic anyway.

They couldn't make it to the summit and back before dark; they decided to camp for the night.

You can also use a semicolon when you join two independent clauses together with one of the following conjunctive adverbs (adverbs that join independent clauses): however, moreover, therefore, consequently, otherwise, nevertheless, thus, etc. For example:

I am going home; moreover, I intend to stay there.

It rained heavily during the afternoon; however, we managed to have our picnic anyway.

They couldn't make it to the summit and back before dark; therefore, they decided to camp for the night.

Use a semicolon to join elements of a series when individual items of the series already include commas.

Recent sites of the Olympic Games include Athens, Greece; Salt Lake City, Utah; Sydney, Australia; Nagano, Japan.

Colon

Use a colon to join 2 independent clauses when you wish to emphasize the second clause.

Road construction in Dallas has hindered travel around town: parts of Main, Fifth, and West Street are closed during the construction.

Use a colon after an independent clause when it is followed by a list, a quotation, appositive, or other idea directly related to the independent clause.

Julie went to the store for some groceries: milk, bread, coffee, and cheese.

BONUS!  Other significant comma rules to remember:

1.      Use commas after introductory a) clauses, b) phrases, or c) words that come before the main clause.

2.      Use a pair of commas in the middle of a sentence to set off clauses, phrases, and words that are not essential to the meaning of the sentence. Use one comma before to indicate the beginning of the pause and one at the end to indicate the end of the pause.

Here are some clues to help you decide whether the sentence element is essential:

·        If you leave out the clause, phrase, or word, does the sentence still make sense?

·        Does the clause, phrase, or word interrupt the flow of words in the original sentence?

·        If you move the element to a different position in the sentence, does the sentence still make sense?

If you answer "yes" to one or more of these questions, then the element in question is nonessential and should be set off with commas. Here are some example sentences with nonessential elements:

Clause: That Tuesday, which happens to be my birthday, is the only day when I am available to meet.

Phrase: This restaurant has an exciting atmosphere. The food, on the other hand, is rather bland.

Word: I appreciate your hard work. In this case, however, you seem to have over-exerted yourself.

3.      DO NOT use commas to set off essential elements of the sentence, such as clauses beginning with that (relative clauses). That clauses after nouns are always essential. That clauses following a verb expressing mental action are always essential.

 

4.      Use commas to separate two or more coordinate adjectives that describe the same noun. Be sure never to add an extra comma between the final adjective and the noun itself or to use commas with non-coordinate adjectives.

Coordinate adjectives are adjectives with equal ("co"-ordinate) status in describing the noun; neither adjective is subordinate to the other. You can decide if two adjectives in a row are coordinate by asking the following questions:

·        Does the sentence make sense if the adjectives are written in reverse order?

·        Does the sentence make sense if the adjectives are written with and between them?

If you answer yes to these questions, then the adjectives are coordinate and should be separated by a comma. Here are some examples of coordinate and non-coordinate adjectives:

He was a difficult, stubborn child. (coordinate)

They lived in a white frame house. (non-coordinate)

She often wore a gray wool shawl. (non-coordinate)

Your cousin has an easy, happy smile. (coordinate)

The 1) relentless, 2) powerful 3) summer sun beat down on them. (1-2 are coordinate; 2-3 are non-coordinate.)

The 1) relentless, 2) powerful, 3) oppressive sun beat down on them. (Both 1-2 and 2-3 are coordinate.)