When deciding whether or not to put a comma before a phrase beginning with the word "as," does one need to consider rules of grammar? And which ones? For instance, whether or not the phrase is restrictive. Here's an example of the kind of sentence I struggle with.: "Peals of delighted laughter burst forth, and tears formed at the corners of his eyes as he watched the children fight." Do I need a comma before the phrase "as he watched..."? If someone could explain this question or direct me to somewhere that could answer my question, I would appreciate it. thanks.