Should I use a comma before “and” or “or”?

Question

Is using a comma then an "and" or an "or" after it good grammar?

Example:

I fell over, and hurt my knee.

Should I go, or not?

Answer 1

Whether it is correct to use a comma after a coordinating conjunction ("and", "but", "or", "nor", "for", "yet", "so") depends on the situation. There are three primary uses of conjunctions:

1. When a coordinating conjunction is used to connect two independent clauses, a comma is always used. Examples:

   • I hit my brother with a stick, and he cried.
   • The rain stopped, and the sun came out again.
   • Should I eat dinner, or should I play a game?

2. When a coordinating conjunction is used to connect a dependent clause, a comma is never used. This includes both of your given examples. Other examples:

   • The boy ran to his room and cried.
   • Frank is a healthy and active child.
   • Should I eat dinner or play a game?

3. When a coordinating conjunction is used to connect three or more items or clauses, a comma is optional (though I personally prefer to use one). Examples:

   • I bought cheese, crackers, and drinks at the store.
   • Should I eat dinner, play a game, or go to the store?

Answer 2

Getting comma usage with and and or can be tricky sometimes, and even the best of us will mess it up once in a while.

Here's a good guide: http://www.grammarbook.com/punctuation/commas.asp

And some examples:

• I fell over and hurt my knee.
• I fell over, hurt my knee , and got stitches.
• I like the colors red and blue.
• I like the colors red, blue , and purple.
• You can buy an apple, a pear , or an orange.
• I can go or stay - which would you prefer?

When used in a list, and and or never take a comma when the list has two items; when it has more, it is generally a good idea to use the comma.

When used as a conjunction it is generally a good idea to use the comma (unless the phrases are very short).

Answer 3

"I went to Japan and returned in 2009" is correct but rather unspecific. It could imply either "I went [say, in 2003] and returned in 2009" or "I went and returned in 2009". The comma in "I went to Japan, and returned in 2009" points to the former; With two commas, "I went to Japan, and returned, in 2009" definitely implies the latter.