First off, all the memorizing you did will not go to waste. While you may not remember all the things you learned, if you run into them again you will be familiar with them. Now if as you do practice questions you realize you don't know enough about a particular rule or concept because you didn't get a certain question right, you can go back and study that rule or concept again, and having seen it in context and felt the need to know about it, you will be more likely to remember it and be able to apply it next time you run into it.
As far as key concepts go, it's a little challenging to know what to list and what not to list, but here are some things you definitely need to understand or be able to catch.
Parallelism or lack of parallelism - This is partly something you can notice via noticing problems with meaning. If a list is not parallel, often the list will convey something that does not make sense.
Placement of modifiers - When modifiers are not placed well, the sentences they are in do not really make sense. You need to be able to notice this.
Subject verb agreement - This is basic. Often subjects and verbs are separated by other words so the the lack of agreement is obscured.
Uses of verb tenses - You need to be clear about what each tense best conveys and when it is best to use certain tenses.
Pronoun rules - You need to know how to place pronouns and how and when to make sure they agree with and refer to nouns.
Certain idioms - There are many idioms that may show up, though from what I understand and have seen they don't show up as much as they used to. At least know like versus as, if versus whether, compared with versus compared to, the correct uses of quantity words, and maybe a dozen other key idioms and conventions. Often even if you have not memorized an idiom you can either find another decision point and avoid having to worry about the idiom or sort out which form is best using logic and meaning. So while once in a while it may be necessary to know a certain idiom to get a question right, that does not happen very often.
Maybe the most important thing for getting SC questions right is being good at seeing the flaws in sentence construction that make it so that a sentence conveys an illogical or ambiguous meaning. Often these flaws are not really easily categorizable, and often sentences can be basically grammatically correct and still convey illogical or ambiguous meanings. If you can see that these types of things are going on, you will tend to do pretty well at SC. If you don't see these types of things, possibly because you are focusing to much on finding grammatical errors, then you will have trouble achieving a high SC hit rate. There are SC questions of which someone focusing solely on grammar might say that most of their answer choices seem right. The only things separating the right answer from the others are subtle issues with what is being expressed by the sentences created using the non right choices.
Regarding hacking skills, I guess what I am referring to comprises analytical skills, a certain amount of flexibility in how one approaches the questions, and skill in figuring out ways to get to right answers.
For instance, one key hacking skill is learning to find decision points that you are confident about. Often a person will see what seems to be an issue with an SC answer choice but not be sure which version is best. A non hacker might impulsively use that issue to separate answer choices, but what you probably would be better off doing is looking for another decision point of which you are more confident.
Another key hacking skill is knowing when to notice that a rule you think is a rule is not actually a rule all the time. People can go through a guide and then think that GMAT SC is about looking for grammar rule violations. Does that make sense? Is grammar so important in business school? Not really. So looking for grammar rule violations only goes so far and often in SC questions there are structures that seem like grammar violations but are not really.
For example, supposedly there is a rule that pronouns need to have a clear referent. However there are instances, such as the following two, where the pronoun it is used in ways in which it does not clearly refer to anything.
It seems that John was here.
It's raining.
I have seen people seek to use the clear referent "rule" to eliminate choices resembling those two examples, and doing that type of thing does not work out. You have to be flexible and see when something that you believe to be a rule does not apply in a particular instance.
Another key hacking skill is being good at comparing answer choices. Maybe you can eliminate a few based on grammar issues, but then you are left with two or three that all seem ok in a way. So now you need to be good at figuring out the key differences that make one the best. You need to be good at looking at the sentence created using an answer choice from end to end to see what's going on and what it conveys. That's not really a skill you develop by reading about a bunch of rules. You have to just work at it and develop an eye for these things.
Another thing that people do is cut out pieces of sentences in order to see more clearly what is going on. Maybe a modifier can be left out and once you do that you see that a subject and verb do not agree.
Another one of my favorites is plugging in words. This is like plugging in numbers in quant problems. If I am not sure about the relative validities of sentence structures, I try using the same structures with different words. Often in so doing I make clear to myself which structure makes sense and which don't.
Overall the idea is to get good at doing whatever it takes to get sentence correction questions right. Often if you don't get one right, the question to ask yourself is, "Is there a way I could have figured out which answer choice is best given what I already know?" Sometimes the answer is that that would be fairly difficult or nearly impossible, but often the answer is a resounding "yes." You could have played with the answer choices, noticed meaning conveyed, or compared things somehow and gotten to the right answer, and that is the type of hacking skill I am talking about.
