Final answer:
The subject in the sentence 'The sun set as the moon began to rise' is sun.
Step-by-step explanation:
The subject in the sentence 'The sun set as the moon began to rise' is sun. The subject of a sentence is the noun or pronoun that performs the action of the verb or is being described.
In this sentence, the sun is the noun that is performing the action of setting. It is the subject of the sentence.
The moon, on the other hand, is not the subject but rather the object of the preposition 'as'. It is not performing the action of rising, it is being described as rising.
Learn more about Subject in a sentence