IELTS grammar: Parallel Structure

Parallel structure means that items joined in a list, a comparison, or a pair of linked clauses share the same grammatical form. It sounds like a minor stylistic preference, but it is graded under Grammatical Range and Accuracy, and it is one of the few error types that an otherwise strong writer can still produce without noticing, because every individual word in the sentence is spelled and used correctly — the fault sits in how the pieces fit together, not in any single piece. A sentence like "The policy aims to reduce costs, increase efficiency, and improve safety" lines up three verb phrases in identical form: verb plus object, repeated three times. Change the third item to a noun phrase or a full clause instead — "The policy aims to reduce costs, increase efficiency, and safety is improved" — and the sentence becomes lopsided even though every word remains grammatical on its own. Examiners read for exactly this kind of structural consistency, because it signals genuine control over how a sentence is architected, not just an ability to string correct words together in sequence. A candidate who writes fluently but inconsistently across lists is, in effect, showing that longer sentences are being assembled rather than planned.

The most common site of the error is a list embedded inside a single sentence, which happens constantly in Task 2 whenever a candidate lists reasons, benefits, causes, or solutions — precisely the kind of sentence where a strong candidate tries to sound sophisticated by cramming in several related ideas at once. "The benefits include lower emissions, reduced traffic, and it saves money" breaks parallel structure: the first two items are noun phrases ("lower emissions," "reduced traffic") and the third suddenly shifts into a full clause with its own subject and verb ("it saves money"). The fix is to keep every item in the same form throughout — "lower emissions, reduced traffic, and lower running costs" — or, less commonly, to convert every item into a clause instead, so long as the choice is applied consistently. Either direction works structurally; what actually fails a sentence is switching form partway through the list. The identical problem appears with verbs after a shared modal: "Governments should raise taxes, cutting subsidies, and to invest in public transport" mixes a base verb, a gerund, and an infinitive in one list, where all three should appear as base verbs following "should": "raise taxes, cut subsidies, and invest in public transport."

Correlative conjunctions — paired connectors like "either...or," "neither...nor," "not only...but also," and "both...and" — carry their own strict parallel requirement: whatever grammatical unit follows the first half of the pair must be mirrored exactly after the second half. "The proposal is not only expensive but also it is impractical" is wrong because "expensive" (an adjective) is paired against a full clause ("it is impractical"); the correct form is "not only expensive but also impractical," adjective matched cleanly to adjective. Similarly, "The reform will either reduce inequality or it might increase social tension" should read "either reduce inequality or increase social tension," verb phrase matched to verb phrase, dropping the redundant second subject and modal. This error is common precisely because candidates usually get the correlative words themselves in the right place, but then let the grammatical form drift between the two halves — a more subtle failure than an obviously missing word, and one that a quick self-check specifically for correlatives will catch reliably.