Choose the correct form: Only after the operation ____ the true cause.
Correct answer: B. did they discover
- A. they discovered
- B. did they discover
- C. they did discover
- D. discovered they
Explanation
When a restrictive expression such as only after, only then or not until opens the sentence, the subject and auxiliary invert as in a question. Without the inversion the sentence sounds ungrammatical in formal writing. The same applies to little did he know.
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About Sentence Inversion
Inversion changes the usual subject-before-verb order, often placing an auxiliary verb before the subject. The topic covers question formation, negative or restrictive adverbials such as never and rarely, conditional inversion, and emphatic structures, while distinguishing inversion from passive voice and ordinary emphasis.
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