Commas seem to be the most troublesome punctuation problem for students. They either put them randomly throughout a paper or avoid using them altogether. Some basic comma rules are helpful: 

A comma is needed when you join two independent clauses (clauses that make a complete thought and can stand alone) with one of the following words:                     

and
or
but
for
nor
yet
so

Incorrect: Tutors are trained to help students with grammar problems; but they are also required to have some computer knowledge.  

Correct: Tutors are trained to help students with grammar problems, but they are also required to have some computer knowledge.

In this example, what comes before and after the comma can stand alone as a complete thought. One way to help students fix this error is to make a diagram showing them the two independent clauses (they can actually be sentences on their own) connected by one of the above conjunctions. On your diagram, you can also show them other ways to correct the sentence: you can make two sentences, or you can remove the conjunction and add a semi-colon.  

Use commas to set off nonrestrictive (nonessential) elements in a sentence. Do not use commas when using restrictive (essential) elements.

Correct: Her husband, who is a teacher, was able to help her with the essay.

You can use commas here because “who is a teacher” can be removed without restricting the meaning of the sentence.

Incorrect: Her husband who is a teacher was able to help her with the essay.

By leaving out the commas here, the writer has created confusion. Does the woman have more than one husband—one who is a teacher and one who is not? 

To help a student with this problem, read the sentence without the nonrestrictive/restrictive element. Is the idea complete? Does the sentence make sense?

Use a comma to set off introductory clauses.

Correct: Although I had seen the Eiffel Tower before, I was again amazed at the size and complexity of the structure.

When you introduce a sentence with a long clause, it is helpful to tell the reader to pause. This enables her to understand the sentence without rereading it. This is important when you are shifting from an introductory statement to the focus of the sentence, as in the above example.

Do not use a comma when a semi-colon is needed (comma splice):

Correct: William Faulkner is a well-known Southern writer; his works revolve around a small town in Mississippi.

A comma instead of a semi-colon would be incorrect here because a comma is not strong enough to connect two independent clauses without a conjunction.