***** NOT A TEACHER *****
(1) You have asked a great question.
(2) It seems that in English, the part of speech often depends on
how it is used in a particular sentence.
How does the dog run? It runs fast.
Therefore, it is an adverb that modifies the verb.
What kind of dog is it? It is a fast dog.
Therefore, it is an adjective that modifies the noun.
I eat too fast. (adverb)
Do you like fast food? (adjective)
(3) More examples:
Martha works hard. / Martha's science class is hard. / Martha has a
hard science class. (The first "hard" is an adverb, and the others are