Question#1Originally Posted by Taka
"tdol, the editor of usingenglish.com, is working hard."
You are right - "the editor of usingenglish.com" is an appositive and as such, it is set off by commas.
Your second example is incorrect. It should be "English and psychology are my favorite subjects". You never use a comma if you mention just two items.
In the text, "the substance of expression" is an appositive or, in another words, a restatement of "sound". In this phrase "substance" means "essential nature" or "essence".