What is the difference between these two sentences: (adjective x adverb)
I was doing an online english test and I had to answer these two questions, actually I had to complete these questions:

1) He's a very bad driver.
He drives very ....

a) dangerously (According to site this is the right answer)
b) dangerous
c) bad ( I selected this one, but the site said it's wrong)

2) And he's a fast driver.
Yes, he drives very ....

a) faster
b) fastly
c) fast (Following the same logical I selected the adjective and the site said I was right)

For me, those questions sound as the opposite from each other, so why can't the both be answered just with an adverb or just an adjective? Why do I have to answer the first with an adverb and the second with an adjective? Is there a tip to help me identify when I have to use each one?
Or is the website wrong?

Question

What is the difference between these two sentences: (adjective x adverb)
I was doing an online english test and I had to answer these two questions, actually I had to complete these questions:

1) He's a very bad driver.
    He drives very ....

a) dangerously (According to site this is the right answer)
b) dangerous 
c)  bad  ( I selected this one, but the site said it's wrong)

2) And he's a fast driver.
Yes, he drives very ....

a) faster 
b) fastly 
c) fast  (Following the same logical I selected the adjective and the site said I was right)

For me, those questions sound as the opposite from each other, so why can't the both be answered just with an adverb or just an adjective? Why do I have to answer the first with an adverb and the second with an adjective? Is there a tip to help me identify when I have to use each one?
Or is the website wrong?

Answer

If you're not sure how to use adjectives and adverbs, check a grammar book.  Grammar in Use by Murphy is a famous grammar book which can be used for self-study, for practice, and later, for review.

He is a dangerous driver.  (dangerous - the adjective modifies the noun driver)
He drives dangerously.  (dangerously - the adverb modifies the verb drives)

fast - adverb and adjective
He is a fast driver.
He drives fast.

Answer

The reason is that "fast" can be both an adjective and an adverb -— in this case, it's an adverb. If you say "I have a fast car," that's an adjective.