Should I use a comma before "and" or "or"?

Is using a comma then an "and" or an "or" after it proper punctuation? Example:

• I fell over, and hurt my knee.
• Should I go, or not?

Whether it is correct to use a comma before 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?

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).

"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.