Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021
Use commas to set off transitional expressions and other word groups
The comma
Punctuation
Transitional expressions
Transitional expressions serve as bridges between sentences or parts of sentences. They include conjunctive adverbs such as however, therefore, and moreover and transitional phrases such as for example, as a matter of fact, and in other words. (For complete lists of these expressions, see 35b.)
When a transitional expression appears between independent clauses in a compound sentence, it is preceded by a semicolon and is usually followed by a comma. (See 35b.)
When a transitional expression appears at the beginning of a sentence or in the middle of an independent clause, it is usually set off with commas.
EXCEPTION: If a transitional expression blends smoothly with the rest of the sentence, calling for little or no pause in reading, it does not need to be set off with a comma.
Aidy’s bicycle is broken; therefore you will need to borrow Saya’s.
Parenthetical expressions
Expressions that provide only supplemental information and interrupt the flow of a sentence should be set off with commas.
Absolute phrases
An absolute phrase usually consists of a noun followed by a participle or participial phrase. (See 49d.) These phrases should be set off with commas.
NOTE: Do not insert a comma between the noun and the participle in an absolute construction.
Word groups expressing contrast
Sharp contrasts beginning with words such as not, never, and unlike are set off with commas.