Hi Vikram,
Knowledge of a rule is not the same as applying the rule. For example take this question on Subject Verb agreement. Now sample these statements:
The direction in which the Earth and the other solid planets—Mercury, Venus, and Mars—spins were determined by collisions with giant celestial bodies in the early history of the Solar System.
The direction in which the Earth and the other solid planets—Mercury, Venus, and Mars—spin were determined by collisions with giant celestial bodies in the early history of the Solar System.
The direction in which the Earth and the other solid planets—Mercury, Venus, and Mars—spins was determined by collisions with giant celestial bodies in the early history of the Solar System.
The direction in which the Earth and the other solid planets—Mercury, Venus, and Mars—spin was determined by collisions with giant celestial bodies in the early history of the Solar System.
The subject "the direction" corresponds with "was determined" and the subject "earth and other solid planets" corresponds with "spin". Tricky right?
However even application of the rules can be less complex. GMAT can get really nasty at the higher level, where even knowledge might not be enough.
Here is the "totem pole" of SC:
Level 1: Applying basic rules of grammar such as SVA, Parallelism etc.
Level 2: Applying more complex rules such as tenses,
Level 3: Identifying incorrect idioms, active-passive
Level 4: Concision, clarity, meaning etc
I might have posed more questions than providing answers. However I hope this helps you in the right direction.
Arun