A sentence in GMAT sentence correction needs to be structurally & logically sound. If sentence consists of parts and if these parts are comparable, then parallelism ensures that the comparable parts are structurally and logically similar.
To know that the sentence is testing you on parallelism, look out for parallel markers. Usually, they are common conjunctions such as and, or & but. You will also find other markers such as not only / but also, from / to, either / or, both / and etc.
Once you have identified the parallel markers, you need to find the part of sentences that needs to be parallel and believe me almost anything in a sentence could be parallel ranging from clauses and phrases to nouns and verbs.
- If there is a list of three or more elements connected by and, the and should be in the list just before the last item, which should be preceded by a comma.
- It is optional to put a comma before and in two clauses connected by and.
- In prepositional phrases, prepositions in parallel phrases do not always have to be same.
- If two parts of sentences which needs to be parallel are clauses, make sure that the clauses starts with same word, even if that makes the sentence appear to wordy.
- Linking verbs are to be treated as parallel markers wherein the subject and the object connected by them needs to be parallel. It is also to be ensured that both sides on linking verbs are parallel in meaning.
- Sometimes, while dealing with verbs & verb forms, it could be disastrous to assume that all of them need to be parallel. Grammatical structure in a sentence is to be checked for logical parallelism before structural parallelism.
In complex questions, there could be sentences with multiple level of parallelism. While dealing with such phrases one needs to identify different levels and ensure that elements or clauses are parallel at each level.