Adverbs vs. adjectives - carefully vs. careful

[Bart]

Test No. errors/inter-8 "Tenses", question 5

There is hardly anything wrong with your answers but you must listen more careful.

(a) hardly
(b) wrong
(c) must
(d) careful

Test No. errors/inter-8 "Tenses", answer 5

There is hardly anything wrong with your answers but you must listen more carefully.

Correct entry: carefully
The error was: (d) careful

Your answer was: n/a
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why do we need an adjective in this sentence?

[Torsten]

Hi Bart,

What we need in this sentence is an adverb at the end: carefully. This adverb describes the verb listen to. You can listen to something carefully.
The only adjective in this sentence is the word wrong.

TOEIC listening, question-response: Does the CEO visit the office often?