Both of the sentences below are correctly punctuated. When the store opens, I will buy groceries.
I will buy groceries when the store opens.

Which answer choice explains why a comma is used in some complex sentences and not in other complex sentences?
Select one:

When a sentence starts with an AAAWWUBBIS word, a comma is probably needed to separate the dependent clause from the independent (main) clause.

When a sentence starts with an AAAWWUBBIS word, it separates two independent clauses.

Commas are not used if a complex sentence begins with a pronoun.

Commas are not used unless the introductory element is a list of phrases that need to be separated for clarity.