50-70% is not necessarily a bad thing. On the real test, most people get about 60% of the problems correct. However, how long are you taking to get that 90% accuracy? Also, why are you practicing untimed in the first place?
Here's what I recommend"
1) Any time you are doing an SC problem for the first time, do it timed. A typical target is 1:15, although you might allow yourself to go slightly over on a longer/more difficult problem. You never want to hit 2:00.
2) Before checking your answer, go back and review untimed. Try to identify all the splits and grammar/meaning issues. If your answer changes, see if you can determine what you were missing the first time and how you might have seen the right answer. Also look at different paths through--on one problem, you might start by looking at the idiom split at the beginning, or you might narrow things down with a subject-verb split first. Which of these paths is the most efficient, and how could you determine that quickly on the exam? This thorough untimed review is where you will do most of your learning.
3) Check your answer & re-review as needed.
As you go through this process, your goal should be to bring your timed and untimed processes closer together. In your untimed review, you're looking for ways to recognize important issues and move more efficiently. In your timed experience, you're trying to think in the same organized way as in the untimed review. You may not always be able to do a perfect job in 1:15, but it should get easier. As for those problems that don't succumb in a reasonable amount of time . . . let them go!
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