4. Placing a comma before a conjunction introducing a coordinate clase.
 The first part of the sentence is a complete sentence when the comma is reached, so that leaves the second clause almost as if it was an after thought. for example:

Even though the dog is green, the cat is black.
Although my neighbor is greedy, she leaves my gold alone.



5.  Do not join independent clauses by a comma.
If two or more clauses are grammatically complete, and are not joined by a conjunction, don't use a comma, use a semicolon. For example:

We understand your situation; we will fix your door.
The dog eats a lot of food; we have to buy the dog more food.