Part 4 transformation: the word limit versus the inversion
A CPE candidate in a timed mock sits Reading and Use of English Part 4. Item 4 reads: He only realised how serious the leak was when the floor began to buckle. The key word is ONLY. The candidate produces three versions while practising, and the teacher must mark them against the published two-mark scheme, which separates correct use of the key word and preservation of meaning from grammatical accuracy. Version A: Only when the floor began to buckle did he realise how serious the leak was. Version B: Only when the floor began to buckle he realised how serious the leak was. Version C: He only realised how serious the leak was when the floor began to buckle. The candidate argues that B is basically the same and that C should pass because it contains the key word. The teacher has 90 seconds to give feedback and wants the candidate to internalise a transferable check, not just correct this item.
1. Award marks to A, B and C and explain the decisive grammatical feature.
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A gets 2. It preserves meaning, uses ONLY correctly, and inverts subject and auxiliary in the main clause: did he realise. B gets 1 at most: meaning and key word are intact, but it lacks inversion. After a fronted restrictive adverbial such as only when, the main clause must show subject-auxiliary inversion; the subordinate when-clause remains in normal order. B reads like an unmarked declarative and is ungrammatical in this frame. C gets 0: it merely repeats the prompt, so the key word has not been used to form a new structure. The decisive test is constituency: identify the main clause after the fronted adverbial, then check whether an auxiliary precedes the subject. If there is no auxiliary, insert one (did) and use the bare infinitive. This is not a style preference; it is the structure the item is testing.
2. The candidate says B is basically the same. Diagnose the error and give a ten-second self-check that transfers to other negative adverbials.
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The candidate is processing meaning but not the syntactic trigger. B is semantically equivalent but grammatically not interchangeable, because the fronted restrictive adverbial changes the word order of the main clause. Ten-second self-check: 1) cover the front adverbial; 2) find the main clause; 3) ask whether a negative or restrictive adverbial here forces an auxiliary before the subject; 4) if yes, rewrite with the auxiliary. Test the rule with Only after, Not until, Under no circumstances and Rarely. The candidate can also paraphrase with It was only when ... that he realised; if the paraphrase is grammatical, the fronted version needs inversion. That transfers because the trigger is the fronted negative or restrictive element, not the word ONLY by itself.