When it comes to GMAT SC it seems as if the list of rules that you need to keep in mind is unending. This post adds to the list of rules but please remember that when you read a sentence your first instinct should not be to fix it unless there is rule that is blatantly broken, always use the 3-2 SPLIT discussed here and here to first identify the error you need to fix.
How to improve your accuracy on GMAT Reading Comprehension
One of the most important skills needed to master GMAT Sentence Correction is the ability to read for the structure and not the content of the sentence. Most of the time test-takers focus on trying to identify a specific rule (that they have learnt) that the sentence is breaking. The exercise of reading the sentence thus becomes similar to going over the words from left to right using a microscope that magnifies grammatical errors.