Extract:
Which word is used incorrectly in the following sentence?
If you calculate too speedy, you may possibly fail to get the right answer.
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A
calculate
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B
speedy
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C
possibly
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D
right
The word "speedy" is used incorrectly; the adverb "speedily" or "quickly" is required to modify the verb "calculate."
A) calculate
"Calculate" is correctly used as base verb form after modal "to." No error affects this verb.
B) speedy
"Speedy" is an adjective, cannot modify verbs. Adjectives modify nouns; adverbs modify verbs. Correct form is adverb "speedily" or synonym "quickly" ("calculate too quickly").
C) possibly
"Possibly" is correctly used as adverb modifying "fail." No error affects this adverb.
D) right
"Right" is correctly used as adjective modifying "answer." No error affects this adjective.
Conclusion
Adjectives cannot modify verbs, only adverbs can. Since "speedy" attempts to modify the verb "calculate," the adverbial form "speedily" (or preferred synonym "quickly") is grammatically required for proper modification.