Find the adverbs and the word (s) it modifies in each sentence:

Bloodhounds can smell quite keenly.

The sense of smell is considerably less keen in humans.

Sometimes smell alerts us quickly to danger.

Have you ever smelled a fire somewhere?

The sense of smell is very closely related to the sense of taste.

Bloodhounds can smell quite keenly.

Quite is an adverb that modifies keenly because it describes how. Keenly is an adverb that modifies smell because it describes how a bloodhound smells. Adverbs modify adjectives, adverbs, and verbs.

The sense of smell is considerably less keen in humans.

Considerably is an adverb that modifies less. Less is an adverb that modifies the adjective keen. In this sentence, considerably and less describe how much.

Sometimes smell alerts us quickly to danger.

Quickly is an adverb that describes how fast we are alerted to danger.

Have you ever smelled a fire somewhere?

Somewhere is an adverb telling where you may have smelled a fire. Ever is an adverb asking about time.

The sense of smell is very closely related to the sense of taste.

Very and closely are both adverbs. Very describes closely by saying how closely. Closely describes how they are related.