It is important to remember that commas make a writer's meaning clear.  When you speak, your tone of voice and pauses punctuate what you're saying, but when you write, your readers have only the printed page to use to understand your meaning.  Punctuation marks, like commas, help the reader get the meaning from your writing.  This is especially true in the case of compound sentences.

A compound sentence is two or more complete, simple sentences that are joined together by a conjunction Conjunctions are words like and, but, or, nor, so, for, yet. For a compound sentence to be punctuated correctly, there must be a comma before the conjunction

Look at these two simple sentences below.  Since they are so closely related they can be joined together with a comma and the conjunction so to form a correct compound sentence.

I slept through my alarm this morning.

I had to run my fastest to catch the school bus.

   I slept through my alarm this morning, so I had to run my
   fastest to catch the school bus.

The reason for this is that a comma is not a strong enough mark of punctuation to separate complete sentences.  Commas can only be used to separate words, ideas, and phrases.

Use a comma before the conjunction (and, but, or, nor, so, for, yet) that joins the two or more clauses (simple sentences) in a compound sentence.

Two simple sentences joined by a conjunction become a compound sentence.  Remember, the comma always comes before the conjunction.

I would have said hello to her, but I didn't recognize her in time.

        There are two simple sentences here joined by the conjunction but.  This
        requires a comma before the but to prevent a run-on sentence.

The firemen rushed to the scene, and they put out the fire.

The traffic is heavy at rush hour, and the most accidents occur from four to six o'clock in the afternoon.

Here you have two simple sentences joined by the conjunction and. These require commas  before the and to prevent a run-on sentence.

Be careful not to get confused by a sentence that has a compound verb.  This type of sentence has only one subject but two verbs.  Since it is just one simple sentence it does not require a comma before the conjunction

For example:

Jim entered the track meet and won the 400 meters.

This sentence has one subject (Jim) and two verbs (entered and won), so it does not need a comma before the and because it is not a compound sentence.

My mother cooked and baked for days getting ready for the family reunion.

This sentence also has one subject (mother) and two verbs (cooked and baked), so it does not need a comma before the and because it is not a compound sentence.

 

Read, think carefully, and remember:

A compound sentence is two or more complete, simple sentences that are joined together by a conjunction
For a compound sentence to be punctuated correctly, there must be a comma before the conjunction

 

Click on the Chalk board to practice commas in compound sentences!

 

There are many other important and useful comma rules that you should know to improve your communication when you write.  Follow the links below to the lessons that introduce them:

Commas in Dates, Addresses, & Letters

Commas
in a

Series

 

Commas with Interrupters
 


Commas in Direct Address

 

Commas With Adjectives
 

 

This site is best viewed with Internet Explorer
Copyright 2001-2011 Oswego City School District
New York State Intermediate Test Prep Center

Studyzone.org