Does anybody know a rule for punctuating adverb clauses that follow the
main clause?  Some are set off by commas and some aren't.  It seems to me
that there are certain subordinating conjunctions with which you hardly
ever use commas, including the biggies "when" and "if."  What about
"because"?  Can we say that some adverb clauses have a restrictive quality
in that they identify a situation in which the main clause is true ("We
cook vegetarian food when Nancy is visiting") and that that explains why
they are not set off by commas?