Hi Anshul Kapoor,

Recent studies indicate that the ability of a soldier to remain calm under attack by enemies, internal or external, determine whether the soldier will be the victor or the vanquished.

A sentence may have more than once clause. In that case, the number of subject verb pair is equal to the number of clauses present in the sentence because a clause must have subject and a verb. So let us break down this sentence into clauses.

Cl 1: Recent studies indicate that
Here "studies" is the subject and "indicate" is the verb. The number of the sv pair agrees with each other. Hence, there is no error in this clause.

Cl 2: the ability of a soldier to remain calm under attack by enemies, internal or external, determine
Here "the ability of the soldier" is the subject and "determine" is the verb. Since "ability" is head of the noun phrase that makes the subject, it will decide the number of the verb. Since "ability" is singular, use of plural verb "determine" is incorrect. It should be singular verb "determines".

Cl 3: whether the soldier will be the victor or the vanquished.
Here, "the soldier" is the subject and "will be" is the verb. Since the verb is in simple future, there is no issue regarding the sv number agreement.
Hence, figure out the sv pairs to identify the clause in a sentence and then check whether the number of the subject agrees with the number of the verb. This is the best way to check this error in sentences with multiple clauses and hence multiple sv pairs.

How to identify clause has been covered in details in Sentence Structure I and II in the free preview concepts section of www.e-gmat.com. All you need to do is register and learn. Registration is free.

Hope this helps.
Thanks.
Shraddha