Harbrace 12b: A comma usually follows introductory words, phrases, and clauses.
Adverb clause, independent clause.
Introductory phrase, subject + predicate
Introductory word, subject + predicate
(1) Adverb clauses before independent clauses
When you write, you make a sound in the reader's head.
-- Russell Baker
[Rule of thumb: always use a comma with an introductory group of words that has a verb form in it.]
(2) Introductory phrases before independent clauses
From the deck, I could not see my father, but I could see my mother facing the ship, her eyes searching to pick me out.
-- Jamaica Kincaid
Omit the comma after introductory prepositional phrases when no misreading would result.
[Rule of thumb: use a comma after an introductory phrase of four words or more.]
(3) Introductory transitional expressions, conjunctive adverbs, interjections, and an introductory yes or no.
Furthermore, benefits include maternity leave of eight weeks . . .
Well, move the ball or move the body. -- Allen Jackson
Yes, I bought my tickets yesterday.