Conjunctions, when they are separated from the principal clauses that depend on them, or when they introduce examples, are generally set off by the comma; as, "But, by a timely call upon Religion, the force of Habit was eluded."—Johnson.
"They know the neck that joins the shore and sea, Or, ah! how chang'd that fearless laugh would be."—Crabbe.
RULE XIV.—PREPOSITIONS.
Prepositions and their objects, when they break the connexion of a simple sentence, or when they do not closely follow the words on which they depend, are generally set off by the comma; as, "Fashion is, for the most part, nothing but the ostentation of riches."—"By reading, we add the experience of others to our own."
"In vain the sage, with retrospective eye,
Would from th' apparent What conclude the Why."—Pope.
RULE XV.—INTERJECTIONS.
Interjections that require a pause, though more commonly emphatic and followed by the ecphoneme, are sometimes set off by the comma; as, "For, lo, I will call all the families of the kingdoms of the north."—Jeremiah, i, 15. "O, 'twas about something you would not understand."—Columbian Orator, p. 221. "Ha, ha! you were finely taken in, then!"—Aikin. "Ha, ha, ha! A facetious gentleman, truly!"—Id.
"Oh, when shall Britain, conscious of her claim, Stand emulous of Greek and Roman fame?"—Pope.
RULE XVI.—WORDS REPEATED.
A word emphatically repeated, is generally set off by the comma; as, "Happy, happy, happy pair!"—Dryden. "Ay, ay, there is some comfort in that."—Shak. "Ah! no, no, no."—Dryden.