Comparison as a form of parallelism compares two or more parts of a sentence. On GMAT one should spot signal words which usually indicates a comparison being made, thereafter one should looks for the parts of sentence being compared.
Four most important comparison signal words are as, like, unlike, than. Before moving towards distinction in usage of these words, we should remember that comparisons should be logically and structurally parallel as discussed in the previous post. A check for logical parallelism ensures that a comparison is being made between similar objects (objects that can be compared or comparable) whereas a check for structural parallelism ensure that a similar grammatical structures have been used in the comparison.
Like vs. as
Like is a preposition and it must be followed by nouns, pronouns, or noun phrases. Like could be used to compare two nouns. Like could also be followed by gerunds when they are being used as nouns. Do not use like while comparing two clauses.
As could be used as a preposition where it appears with nouns or as a conjunction where it appears with clauses. For comparison of two clauses, one could correctly use as but not like.
One can omit unnecessary words in second part of the comparison, but should remember that presence of unnecessary words in the second part doesn’t make a sentence incorrect. One need to be careful while choosing between options.
Comparative & superlative forms
Comparative form of adverbs and adjectives should be used when comparison is being made between two things. For comparing more than two things, superlative form of adverbs and adjectives should be used.
Some check points:
- Use comparative adjectives only in presence of than; use of than with than with comparative form makes comparison clear.
- For adverbs ending with –ly, do not make a comparative form by adding – er. More should be used in those cases.
- Some more tips are listed here.