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Put a comma after an adverbial phrase at the beginning of a sentence.

An adverbial phrase is a group of words (without a subject or a verb) that does the work of an adverb. It modifies the verb and answers questions like how? where? when?)

Standard Form: Adverbial phrase, rest of sentence.

Example: In the distant future, we may all be driving electric cars.

Exception 1: If the opening adverbial phrase is very short the comma is optional.

Example: Before dawn we should prepare for our trip.

Exception 2: If the word after the adverbial phrase is a verb, do not use a comma after the phrase.

Example: In the depths of the wilderness lives a ferocious lion.

Use commas to set off nonrestrictive phrases and clauses

These phrases and clauses give information but are not really essential to the meaning of the sentence.

Sample Form: Subject, nonrestrictive clause, predicate.

Example: Katelyn, who is an only child, demands a lot of attention.

Do not use commas around restrictive clauses.

Restrictive clauses provide information essential to the meaning of the sentence.

Sample Form: Subject restrictive clause predicate.

Example: The boy [who broke the window] is at the door.

Use commas to set off nonrestrictive appositives.

An appositive is a noun that comes after another noun and gives additional information about it. An appositive can come in the middle or end of a sentence.

Examples:

Dr. Sogohan, the headmaster of our school, never shouts.
One person who never shouts is Dr. Sogohan, the headmaster of our school.

Do not use commas around restrictive appositives.

Restrictive appositives provide information crucial to the understanding of the sentence and should not be set apart with commas.
Example: The musician Bobby Groffman visited our town.

Put a comma after words like oh, yes, and no at the beginning of a sentence.

Form: Interjection, independent clause.

Example: Yes, I do have what you’re looking for.