The correct answer is A: was. This question tests your understanding of subject-verb inversion, a crucial grammatical structure in English, especially when a negative adverbial phrase like "Not only" initiates a sentence. When "Not only" is positioned at the very beginning of a sentence, it necessitates an inversion of the subject and the auxiliary verb, mirroring the structure typically found in interrogative sentences. The standard pattern becomes "Not only + auxiliary verb + subject + main verb (if applicable)..."
In this particular sentence, the subject is "he," which is a singular pronoun. Furthermore, the phrase "forgot his assignment" clearly indicates that the action occurred in the past tense. Consequently, the appropriate choice is the singular past tense form of the auxiliary verb "to be," which is "was," to ensure agreement with the singular subject "he."
- A: was is correct as it aligns with both the singular subject and the past tense context, while also adhering to the inversion rule.
- B: is is incorrect due to its present tense nature.
- C: were is incorrect because it is the plural past tense form.
- D: are is incorrect as it is the plural present tense form.
The grammatically correct sentence is: "Not only was he late, but he also forgot his assignment."