Ok, let's get one thing straight.

GMAT SC is NOT really testing your knowledge of rules, idioms, or GMAT SC strategies. It's really testing one thing, your ability to figure out what makes sense and what works best, and it is mostly using sentence construction as a forum for testing that one thing.

Get it?

This is a business school entrance exam. Do you think that in business school it particularly matters whether you know that generally a comma precedes which or that the word it in many cases requires a referent? Of course not.

What matters in business school is your skill in grasping problems and in seeing the logic of situations.

So when you are doing SC questions, the types of things that you need to be considering include the logic of the overall situation and which answer choice creates the most effective sentence.

All that stuff you are learning in the Manhattan class is just stuff you may or may not need to notice in the course of considering the above mentioned types of things.

As a matter of fact I am working with a guy right now who is remarkably facile with the English language and so clear in the way that he speaks, and he took a Manhattan Prep class and came out of it looking for "what the SC question is testing", and was getting a LOW hit rate on SC questions. Why? Because he was no longer looking at sentences as structures that are effective or not. He was looking for all these little specific things, the things you are worried you won't see.

After changing his focus to looking at entire sentences and at entire SC answer choices with the intent of seeing which makes the most sense or is most effective, in the last SC practice set he told me about he got all of them right.

So yes, in order to get GMAT SC questions right, you need to notice things like parallelism, subject verb agreement, and placement of modifiers, but for the most part really, without ever having heard of those things you could get the right answers.

This sentence is just plain confusing. Throw mother out the window a bar of soap. Do you need to know that there is something called a misplaced modifier to know that this one is better? Throw a bar of soap out out the window to mother. Not really. Anyone can tell that the second one more clearly conveys that the idea is to throw the soap rather than mother. So while learning what the Manhattan class has to teach can be helpful, the key thing is seeing what's going on, and working or not working, in the sentences and answer choices.

Here is from GMAC itself a blog post that discusses this.

https://www.mba.com/us/the-gmat-blog-hub ... -exam.aspx

Here is a blog post I wrote that discusses some related ideas.

https://infinitemindprep.com/on-the-gmat ... ogic-rule/