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In theory, parallel structure is a relatively easy concept to master: it simply refers to the fact that items in a list, as well as constructions on either side of a conjunction such as and or but, should be kept in the same format (all nouns or all verbs).
In very simple sentences, e.g., I went to bed late but woke up early, this rule is generally quite simple to apply.
When sentences are long and contain a lot of information, however, things get a bit trickier. Keeping forms parallel requires the writer to keep track of and understand how words and phrases in different parts of a sentence relate to one another.
One very common issue involves the use of main verbs after modal verbs such as can, should, or might. As anyone who speaks English at a reasonably high level knows, main verbs are never conjugated in this construction, e.g., one would say it might work, not it might works. But when the two verbs are separated, there’s a common tendency forget about the first one and to stick an -s on the second.
This is an issue that appears in the writing of both native and non-native English speakers, but it’s particularly rampant in IELTS essays. It may also be tested in GMAT Sentence Corrections.
So, consider the following sentence:
A business in the United States can receive tax breaks and other incentives for using renewable energy and also purchases green energy at cheaper prices than traditional fossil fuels.
The word and signals that businesses can obtain two main types of benefits, and that the sentence will present those benefits in a way that is grammatically parallel: the construction on one side of the word and must match construction on the other side.
In the version above, however, the two sides do not match.
To understand the error more clearly, we can break the sentence down as follows. The key is to understand that both verbs must “fit” after the word can, even though the second verb appears much later.
A business in the United States can:
1) …receive tax breaks and incentives
Yes, this works. It is correct to say A businesses in the United States can receive. Because receive is placed immediately after can, any mistake would be obvious.
2) purchases green energy
No, this is not correct because purchases does not fit after can. Instead, we must say A businesses in the United States can purchase green energy. Because purchase is placed far from can in the original sentence, the error is not immediately obvious.
Note that although it is technically acceptable to say a business in the United States purchases green energy, that statement does not make sense in the context of the sentence; the focus is on what a business can do. The no -s form of the verb is used to convey the logical meaning.
So the corrected sentence reads:
A business in the United States can receive tax breaks and other incentives for using renewable energy and also purchase green energy at cheaper prices than traditional fossil fuels.