According to Grammarly's blog, you should use a comma with non-restrictive phrases:
Jeff’s new car, which is less than a month old, already started
leaking oil.
And leave it out with prepositional phrases:
The platform on which we built our program is very stable.
Here's my confusion:
He forgot the present he had bought for Mary, which meant that he had to go back home a second time.
I have no idea what the bolded past is. A non-restrictive phrase? A prepositional phrase? Should there be a comma before it?
Best Answer
In this sentence "which meant that he had to go back home a second time" is a non-restrictive relative clause, and its antecedent is the entire preceding clause "He forgot the present he had bought for Mary." The which refers to what is described by the preceding clause, the situation that "He forgot the present he had bought for Mary." And yes, non-restrictive clauses are usually set off by commas.