[abhisays]

1. The maximum number of mistakes in the original sentence in a Sentence Correction question is 2. That means that some questions contain no mistakes, some contain one mistake, and some contain two mistakes.
2. The mistakes in Sentence Correction questions are never spelling mistakes. Each and every word used in Sentence Correction is an existing correctly spelled word. You won't see things like sleeped or taked and you won't see made up words. On the other hand, there could be punctuation mistakes such as a missing comma.
3. The GMAT does not deal with English mastery - it deals with logic and cognitive skills. Thus, the correct answer choice must be logically correct. A sentence such as "The green dream barks" is grammatically correct but illogical.
4. The correct answer choice corrects whatever mistakes the original sentence had, but does not radically change its overall meaning. So radical changes to the sentence's meaning may also be a reason for eliminating answer choices. Subtle changes of wording, however, are common, and are usually not considered meaning changes.
5. We don't look for a correct answer choice - we eliminate incorrect answer choices and choose the last answer choice standing. Why? Because there are numerous ways of correcting mistakes, and we never know which way will be chosen. If we look for an answer choice that corrects the mistake in a certain way, we might not find such an answer choice, or we might (heaven forbid) eliminate the correct answer choice because it corrects the mistake in a different way