A comma before "not" is used to separate independent clauses or to set off introductory elements, but it's often unnecessary when "not" is part of a negative phrase. The rule depends on context.
A comma is needed before "not" when it begins a dependent clause that modifies the main clause. For example:
I wanted to go, not realizing the risks involved.
Here, "not realizing the risks involved" is a dependent clause introduced by "not," requiring a comma before it.
The two primary scenarios for using a comma before "not" are:
For example:
Not surprisingly, the results were disappointing.
In this case, "Not surprisingly" is an introductory phrase requiring a comma.
Many writers mistakenly place a comma before "not" when it's part of a negative phrase, such as "not only." According to the Grammarly Blog, this is incorrect because "not" functions as part of the phrase, not as a standalone word.
For example:
She got an A.
Here, no comma is needed before "not" because it's part of the "not only...but also" structure.
Consider this sentence:
I thought he would come, not knowing his plans had changed.
The comma before "not" is correct because "not knowing his plans had changed" is a dependent clause introduced by "not." However, in this sentence:
He was not happy with the outcome.
No comma is needed before "not" because it's part of the negative phrase "was not."
When editing a colleague's report, I noticed they added a comma before "not" in the phrase "not only." I pointed out the error, and we corrected it to follow standard grammar rules. This small change improved the clarity of the sentence.