In the Parent's Guide, I talked about how if you have a sufficient command of the English language, when you hear grammar used incorrectly, it stands out to you. You can hear it just like a musician can hear a pitch that is off-key. Bad grammar jumps out to you.
One thing that will help improve your ability to detect incorrect grammar in a testing environment is to sound it out, either silently in your head, or mouthing out the words very quietly to yourself as you read it.
I'm not advocating annoying the other students you're testing with, but this is a valid technique.
Basically, by applying this technique, you will be drawing on all of your memories of hearing English spoken correctly, in addition to reading it. Believe it or not, that can give you a huge edge.
Also, in developing these skills, as you are reading the Elements of Grammar, which is one of the books I recommend in SAT ACT Mastery, you may want to read the examples aloud. Read both the correct and incorrect examples aloud and note the difference. That will help you develop your ear for correct grammar.
Grammar has its roots as a commonly accepted pattern of speaking and writing. The human brain has dealt with spoken word for a lot longer than it's dealt with written word. That's a plausible explanation for why when you're doing work with grammar, when you read it aloud and work through it aloud, you'll have an easier time with it.
In any event, regardless of my theories on the subject, try the spoken approach to grammar and see if it helps.
Read the sentences aloud, and it will become obvious to you where there is supposed to be a break or a comma, etc. etc. There are some things in grammar that are not intuitive like this, where you just have to remember what it looks like, but those are few and far between.
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