which is correct and why?

[Taka]

It is difficult to understand Prof. Goldberg because he speaks ( )
1.quick 2.soon 3.rapid 4.very rapidly

It seems to me that both #1 and #4 work...

[Mister Micawber]

No, you need an adverb; it would have to be 'quickly', Taka.

[Taka]

No, you need an adverb; it would have to be 'quickly', Taka.

Isn't it possible to use "quick" adverbially, as in, say, "Do it quick!"?

[Mister Micawber]

You can say it, but don't write it unless you're recording dialogue, Taka.

[Taka]

You can say it, but don't write it unless you're recording dialogue, Taka.

You mean it's purely colloquial?

[Taka]

Any comments on this one, tdol?

[Casiopea]

No, you need an adverb; it would have to be 'quickly', Taka.

Isn't it possible to use "quick" adverbially, as in, say, "Do it quick!"?

In that particular context (i.e., Do it quick!), it's possible, iff quick refers to 'quick-like'. In that case, 'quick' would be an adjective in form but an adverb in function, a near synonym with "right now, this very instant", like this,

EX: Do it quick(-like)! (i.e., This very instant, now!)

Adverb Test
Do it when/In what manner?
Quick-like (i.e., right now, this very instant).

As for the context in question, though, MM is correct in pointing out that "he speaks _____" requires an adverb,

Adverb Test
He speaks quick.
When/In what manner does he speak?
He speaks quick-like. (i.e., right now, this very instant) (Not OK)

Note, colloq. He speaks *quick ~ He speaks too fast.

He speaks quickly, very rapidly. (adverbs)
In what manner does he speak?

All the best, :D

[Tdol]

In the original example, the use of 'quick' by a native speaker would probably be regional and many would consider it wrong- it's not like 'do it quick'.

[Casiopea]

In the original example, the use of 'quick' by a native speaker would probably be regional and many would consider it wrong- it's not like 'do it quick'.

I couldn't agree more.

Note, colloq. He speaks *quick ~ He speaks too fast.

[Taka]

Thanks! :D