Let me add just briefly to what Wanda has written.  I use this same
distinction in teaching this point.  In addition I suggest that students
think of the sentence as a response to a question.  I ask them to
underscore the part of the sentence that contains the answer to the
question.  If they underscore the "because" portion, they do not need a
comma; if they underscore the main clause portion, they do need a comma.

1.  "Why has Mrs. VG gone home?"  The answer is "... because she became
ill." There is no comma; the clause is essential, for it contains the
answer to the question.

2.  "What has Mrs. VG done as a result of not having her  car in the
parking lot?"  The is "Mrs. VG has gone home ...."  Here a comma is needed;
the clause is non-essential and thus is set off with commas.

A further distinction (which can work with native speakers with a "natural"
ear):  In #1, in which the sentence is is telling why Mrs. VG has gone home
(and the answer to the question is in the "because" clause), the sentence
would "sound funny" if the dependent clause were shifted to the
introductory position.  A native speaker would not move the clause.

However, in #2, in which the sentence is telling what Vrs. VG has done as a
result of the situation mentioned in the "because" clause), the sentence
"sounds fine" if the dependent clause is shifted to the initial position:
"Because ..., Mrs. VG has gone home."  (Of course, some folks object to
using the word "because" to begin a sentence--but that's a different program.)

The same technique/approach works with "since."


At 10:45 AM 5/21/99 +0000, Wanda VanGoor wrote:
>This will probably only add to the confusion, but I've found it very
>useful in dealing with punctuating the "because" clauses.  If the
>"because" clause gives a REASON, no comma is needed.  If the "because"
>clause gives evidence (not reason), use a comma.
>
>      Mrs. VG has gone home because she became ill.
>
>      Mrs. VG has gone home, because her car is not in the parking lot.
>
>In the "evidence" sentence, one can mentally insert "I know that" between
>"home" and "because."  The comma takes the place of the "I know that."
>
>I've never seen it in a text (except my own!)--but it works!
>
>