Use a comma before "as" when it introduces:
* A dependent clause: This is a clause that cannot stand alone as a sentence.
* Example: *She was very tired, as she had been working all day.* ("As she had been working all day" is a dependent clause.)
* A nonrestrictive clause: This is a clause that gives extra information but is not essential to the meaning of the sentence.
* Example: *The dog, as usual, barked at the mailman.* ("As usual" is a nonrestrictive clause.)
Don't use a comma before "as" when it:
* Introduces a phrase that acts like an adjective:
* Example: *He was known as a great musician.* ("As a great musician" modifies "He" and acts like an adjective.)
* Introduces a comparison:
* Example: *The cake is as delicious as ever.* ("As delicious as ever" compares the cake's current deliciousness.)
* Is part of a prepositional phrase:
* Example: *She is known as the CEO of the company.* ("As the CEO of the company" acts as a prepositional phrase.)
If you're unsure, try reading the sentence out loud and see if a pause naturally occurs before "as." If so, a comma is likely needed.