Final answer:
The statement is false. It's important to use commas with coordinating conjunctions when joining two independent clauses, but not necessarily before a dependent phrase following a coordinating conjunction unless it contrasts the independent clause.
Step-by-step explanation:
The statement "Don't use commas before a coordinate conjunction and a dependent phrase" is false. Commas should be used with coordinating conjunctions when they join two independent clauses. These conjunctions include the words for, and, nor, but, or, yet, and so, remembered by the acronym FANBOYS. However, if the phrase following the coordinating conjunction is not an independent clause, a comma is not necessary.
Moreover, a comma should be used after a dependent clause that precedes an independent clause, especially when the subordinating conjunction implies contrast, like though or whereas. An example of this is: If you speak the truth, have a foot in the stirrup. For non-contrast subordinating conjunctions, like because, a comma is generally not used before the dependent clause.