Yvonne, you are correct that context can make a difference in terms of whether or not a relative clause is essential or non-essential. The two sentences I used were taken from an English grammar book as examples, for what it's worth. In many cases, the pronoun that you choose and the use of commas or not will turn the clause into essential or non-essential if that makes sense.
Lynn, the rule about commas with compound sentences is basically this: Use a comma before the conjunction (and, or, but);however, you may leave it out if the two independent clauses are both quite short, i.e. Lois sang and she danced.
Hope this helps.
Ann
