There are almost always multiple rules being tested in a single sentence, so know that from the start. Deal with the original sentence first - if you can find any errors there, cross off A and also cross off any choices that repeat the exact same error.
Once you've exhausted any errors you spot in the original sentence, compare the remaining choices, but compare them VERTICALLY - that is, look for differences in the same parts of each answer choice. The differences tell you what the potential errors might be (though sometimes a difference doesn't actually indicate an error). Start with whatever differences you know are errors and, specifically, start with whichever difference you feel most confident about. Keep doing this till you have only one choice left.
For the subj-verb errors, as I mentioned before, figure out WHY you're making the mistakes you make. Then you can actually figure out how to combat those mistakes.
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Stacey Koprince
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Manhattan GMAT
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