Munhugg gives excellent advice. There are a lot of concepts to learn, but ultimately your task is a search-and-destroy one, not a learn-and-understand-everything-possible one.
That said, if you are lacking in foundational skills it can be really tough to zero in on why something is better/worse than something else. For my students who struggle with the level of content in the strategy guides (and this comes up quite a bit! Grammar is not formally taught in many schools in the US), I give the following advice:
(1) Start with a foundational grammar book BEFORE attacking ANY OG questions. We have a Foundations of Verbal
book, and there are probably other suggestions on these forums as to what has been useful to other people (I'm not familiar with the Kaplan
book mentioned above, but go to a bookstore/Amazon and browse through what's available. Find the book with the style and format that is most accessible to you--grammar can be a tough slog at the beginning, so you don't want to choose material that you're already resistant to.)
(2) Practice whittling down each sentence to its "bones." What parts can you cross off and ignore? What parts are crucial to the structure? Every sentence needs a subject and verb, and you should be able to pick out that subject and verb fairly quickly. You can copy sentences down and literally cross off inessential parts to help train your mind to think this way.
(3) Only after you feel like you have a grasp of the basics, start with your "standard" content materials (for us, it's the Sentence Correction Strategy Guide). Go SLOWLY and practice the content subject-by-subject. Apply that knowledge to OG questions that fall into that category (we provide a breakdown of these to our students, and other companies may do the same). Trying to do too much here can get you in over your head.
This is very important: accept that at the beginning, you will miss questions because you haven't hit all the topics yet! Sentence Correction questions rarely test only one topic--there can be as many as 4 or 5 areas tested in the splits! If you've only worked on subject-verb agreement, and correctly eliminate based on that split, but get bogged down because of a pronoun issue and you haven't had pronouns yet, that's okay. That is, in fact, a cause for celebration! You got the thing that you've studied right, and the thing that you haven't wrong. Pick up the rule that you didn't know, make a note of it, and move forward with confidence. Save that question for later though--make a notecard, or use whatever your preferred system is-- so that when you *have* studied all those content areas, you can go back and practice your sharpened subject verb AND pronoun skills. At some point it will be helpful to get some process help, not just grammar help. It can be with a tutor, a class, a friend, or a book--just make sure you have a clear and repeatable way of attacking questions that uses your time efficiently.
After substantial subject-by-subject studying, start mixing content in drills. It is much harder to solve a question if you don't know exactly what you're looking for. As you're working, make a list of "clues" that help you identify the key issue (for example, "and" is a huge parallelism trigger, etc).
And finally, throughout this whole process--assess assess assess! Burning through questions is the *least* effective way to make progress. Spend just as much time (or more time) reviewing your work and assessing your progress as you do working on actual questions. Be honest with yourself, and never gloss over things you only "kind of" understand.
JP Park | Manhattan GMAT Instructor | Los Angeles
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