If the comma is left out in the textbook example, the antecedent becomes more confusing, because, as you say "I think 'describing' just modifies a letter " Compare with this one without the comma: They wrote the company a letter describing its advertising as inappropriate.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
AnonymousSo you agree that 'describing...' modifies 'a letter' in the sentence? And if it is apart from a real antecedent, it is better to put a comma not to make the sentence confusing? Thank you.So you agree that 'describing...' modifies 'a letter' in the sentence? yes.
AlpheccaStars in this case, yes.Sorry to bother you a lot but I would like to make sure that a comma is needed 100% or optional. I think the meaning of 'in this case' in your reply says a comma is always not needed when a real antecedent is apart from relative pronouns. Did I understand you right? Thank you.