Final answer:
The simple subject in the sentence 'The big gray car drove away' is 'car'.
Step-by-step explanation:
The simple subject in the sentence 'The big gray car drove away' is 'car'. The subject of a sentence is the noun or pronoun that performs the action of the verb or that the verb describes. In this sentence, the noun 'car' is the subject because it is the noun that the verb 'drove' describes.