A word or an expression used independently in addressing a person or an object is separated by a comma from the rest of the sentence.
Example:—Antonio, light my lamp within my chamber.
Adjectival, participial, and absolute phrases are each separated by a comma from the remainder of the sentence.
Example:—Awkward in his person, James was ill qualified to command respect.
Adverbs or adverbial phrases, when used as connectives, or when they modify not single words, but clauses or sentences, are each followed by a comma; and if used intermediately they admit a comma before as well as after them.
Example:—The most vigorous thinkers and writers are, in fact, self-taught.
When a phrase beginning with a preposition, an adverb, or a conjunction relates to or modifies a preceding portion of the sentence, a comma is unnecessary if the parts are closely connected in sense.
Example:—For that agency he applied without a recommendation.
Many phrases which, in their natural or usual order, do not require to be punctuated, are, when placed in some other or unnatural position, set off by a comma from the rest of the sentence.
Example:—By Cowley, the philosopher Hobbes is compared to Columbus.