How do you choose the correct preposition in an idiomatic expression on the ACT, where the right answer is fixed by convention rather than a rule?
Idioms and prepositions on ACT English: choosing the conventionally correct preposition that pairs with a given verb, adjective, or noun (interested in, capable of, different from), recognizing that these pairings are fixed by usage, and using your ear plus common pairings to pick the idiomatic option.
A focused answer to idioms and prepositions on ACT English: choosing the conventionally correct preposition in fixed expressions (interested in, capable of, different from, depend on), why these are set by usage rather than rule, and how to use common pairings and your ear to pick the idiomatic option.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this skill is asking
Some ACT questions test idiomatic expressions: a verb, adjective, or noun that conventionally pairs with a specific preposition ("interested in", "capable of", "different from"). There is no logical rule that predicts the right preposition; the pairing is fixed by usage, so the test is whether you know (or can hear) the standard combination. The skill is recognizing the idiom and choosing the conventionally correct preposition, not the one that seems logical.
Idioms are fixed by usage
Unlike grammar rules, idioms cannot be derived; they are simply the accepted pairings.
The practical consequence: you cannot reason your way to the answer from grammar; you match the expression to the pairing native usage has fixed. Reading widely builds this, but a short list of high-frequency pairings covers most ACT cases.
High-frequency pairings
A compact list handles the idioms the ACT reuses.
Applying it to an underlined preposition
When a preposition is underlined after a verb, adjective, or noun, treat it as an idiom test.
Why ear plus a short list works
Idiom questions cannot be solved by rule, so the method is recognition: hear the natural pairing and confirm it against the common list. Native-like usage handles most cases, but the ACT deliberately offers nonstandard options that "sound okay" to some ears ("different than", "could care less" for "couldn't care less"), so the short list guards against those. This topic overlaps with word choice and tone (idioms are part of natural, register-appropriate expression) and is usually quick once you recognize that the underlined preposition is the thing being tested.
Try this
Q1. Why can't you derive the correct preposition in an idiom from a grammar rule, and how do you choose it instead? [Recall]
- Cue. Idiomatic pairings are fixed by convention, not by any general rule, so logic does not predict them. You choose by recognizing the standard pairing (using your ear plus a learned list of common pairings like "interested in", "capable of", "different from"), not by reasoning from grammar.
Q2. A sentence reads "The new alloy is superior than the old one." Is the preposition idiomatic? Fix it if not. [Short explanation]
- Cue. No. "Superior" idiomatically pairs with "to", not "than": "superior to the old one." Like "different from" and "prefer ... to", "superior to" is the conventional pairing, and "superior than" is nonstandard, so the fix is to replace "than" with "to".
Exam-style practice questions
Practice questions written in the style of ACT exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
ACT English (style)1 marksChoose the best option: 'Her conclusions were quite different than the ones in the original study.' (A) NO CHANGE (B) different from (C) different to (D) differently thanShow worked answer →
The correct answer is (B), "different from". In standard American usage, the idiomatic pairing is "different from". "Different than" is widely considered nonstandard on the ACT, especially before a noun phrase.
Why not the others: (A) "different than" is the nonstandard pairing the question is testing; (C) "different to" is British usage, not standard American; (D) "differently than" wrongly turns the adjective into an adverb. The fixed idiom is "different from".
ACT English (style)1 marksChoose the best option: 'The success of the project depends ___ careful planning and a clear budget.' (A) of (B) on (C) in (D) atShow worked answer →
The correct answer is (B), "on". The verb "depend" idiomatically pairs with "on": you depend on something. This pairing is fixed by convention.
Why not the others: (A) "depends of", (C) "depends in", and (D) "depends at" are all non-idiomatic; English simply does not pair "depend" with those prepositions. The correct idiom is "depend on".
Related dot points
- Word choice and precision on ACT English: selecting the word whose denotation and connotation exactly fit the sentence's meaning and context, rejecting vague or approximately right words, and using surrounding context to pick the precise term in an underlined portion.
A focused answer to word choice and precision on ACT English: choosing the word whose exact meaning and connotation fit the context, telling a precise choice from a vague or approximately right one, and using the surrounding sentence to pick the right term, with a routine for the underlined word.
- Tone and style consistency on ACT English: matching word choice to the passage's established register (formal, neutral, or conversational), rejecting words that clash with the surrounding tone (slang in a formal passage, jargon where plain words fit), and using the passage's own diction as the standard for an underlined choice.
A focused answer to tone and style consistency on ACT English: matching word choice to the passage's register, rejecting words that are too casual or too formal for the surrounding tone, and using the passage's own diction as the standard, with a routine for the underlined choice.
- Commonly confused words on ACT English: distinguishing homophone and near-homophone pairs (their/there/they're, your/you're, its/it's, then/than, affect/effect, fewer/less, who's/whose) by meaning and part of speech, and choosing the spelling that fits the sentence.
A focused answer to commonly confused words on ACT English: telling apart homophone and near-homophone pairs (their/there/they're, your/you're, its/it's, then/than, affect/effect, fewer/less) by meaning and part of speech, with quick tests and a routine for choosing the right word in an underlined portion.
- Concision and redundancy on ACT English: preferring the shortest option that preserves the meaning, spotting redundancy (two words that say the same thing, such as past history) and wordy phrases (due to the fact that for because), and choosing the tight version when grammar and meaning are otherwise equal.
A focused answer to concision and redundancy on ACT English: why the shortest option that keeps the meaning usually wins, how to spot redundancy (past history, close proximity) and wordiness (due to the fact that), and the rule that when options are otherwise equal, the tightest one is correct, with a routine.
Sources & how we know this
- Description of the ACT English Test — ACT, Inc. (2025)
- Preparing for the ACT Test — ACT, Inc. (2025)