1. Careless mistakes. Sometimes this is just "my brain was tired"—but your brain may get tired during the real test, too, right? And, often, the specific careless mistake is tied to something that we can make better for next time.
So think about what bad habit you need to break or good habit you need to build that would help you to better avoid that specific kind of careless mistake in future.
I'm jumping ahead and read the rest of your post now. So I'm going to continue from the end of your post.
When people are also studying for the TOEFL, I generally recommend studying for and taking that test first. As you noted, the TOEFL doesn't get to the same level of intricacy. Mastering what you need for the TOEFL first will then help you in your GMAT studies. Just something to think about.
Next, your items 2 and 3 are also geared towards the overall goal of "How do I get these all right in future?"
We need to talk about that. The goal is NOT to be able to answer everything correctly in future. This isn't a school test—it's a business test. Part of what you're doing is setting priorities, and some priorities are "This is too hard and I just need to figure out how I can know to guess fast on something like this in future."
Halley's Comet is a great example. The underline is super short. There are very few differences among the answers—they can't really "hide" anything in there. The sentence structure is really unusual. It's hard to
articulate what's going on with the differences in the answer choices—even though there are only about 5 words in each choice, so it's really easy to see the differences. You can see what they
are...but you can't really articulate what they
mean.
Those characteristics in general = signals that this is way too hard! Guess and move on. And don't even try to study it to get better—not when you've got lower-hanging fruit to go after, such as careless mistakes or things that you could spot if you trained yourself to pay more attention to meaning, etc.
Which brings us to #2: You seem to be saying two different things here. First, you say that you didn't fully understand the sentence. (Not understanding the sentence, by the way, is another reason to bail / guess and move on.)
But then you say that you just forgot to consider meaning because you were so focused on grammar. If it was the case that you did understand but just forgot to consider meaning, but
once you did think about meaning you were able to see the issue, then the real problem here is that we have to train you to remember to consider meaning.
And you do that just like you train yourself to do anything else: building an explicit, conscious habit to do something. In our SC process, the second step is Read For Meaning.
Maybe you need to make a flash card that says MEANING! in capital letters on it and prop it up every time you're doing an SC.
Maybe, while you're reading the original sentence, you hold that flash card in your hand—to remind yourself that you have to consider meaning, every time.
Maybe you review problems that you've already done and try to call out ALL potential meaning issues in all answer choices, even if you were able to eliminate an answer using grammar. Then check the solution (or our Navigator solution for OG) to see whether we called something out as a meaning issue that you didn't notice for that reason.
Maybe you try a new SC untimed and you *first* have to look for any and all meaning issues before you're allowed to use any grammar issues you spot. Jot down the pure grammar clues but tell yourself you just can't deal with them quite yet. First, you have to consider whether anything's illogical, or redundant, or ambiguous.
Maybe...brainstorm some other things you could try that would get your brain to focus on meaning first. (This doesn't mean that, in future, you won't use grammar—you will, and you'll even use grammar first a lot of the time. But for now, you need to train yourself to focus on meaning first because the issue is that you're forgetting to use it at all. Once it's ingrained in your brain that you will always consider meaning alongside of grammar, then you can choose which issues to deal with first.)
