When I was studying for my CELPIP exam I got material from them. Unfortunately, I can not put the PDF to the public because it has a watermark with my personal email. But I can put you here what it says.
CELPIP's second error as they state:
We see a lot of run-on sentences in the written part of the CELPIP. A run-on sentence is composed of two independent (i.e., potentially stand-alone) clauses that have been stuck together with either commas or nothing at all.
There are two types of run-on sentences: comma splices and fused sentences. A comma splice is two independent clauses joined only by a comma. (Thank you for your resumé, we will contact you soon.) A fused sentence is two independent clauses joined with nothing at all. (We have selected the candidates to be interviewed we would like you to come in next week.) The easiest solution is to break the clauses into separate sentences or else coordinate them.
In repairing a run-on sentence, we also have the option of changing one of the independent clauses into a dependent clause by using an appropriate subordinating word (e.g., because, if, although). The result is a complex sentence with one dependent clause and one independent clause.