Final answer:
In the sentence provided, 'Sean' is the subject, as it is the noun that is performing the action of beginning to shiver, with 'began' being the main verb.
Step-by-step explanation:
The subject of a sentence is the noun or pronoun that the sentence is about; it's often the entity that performs the action of the verb. In the sentence 'Sean began to shiver at the thought of going back outdoors,' the subject is Sean because Sean is the person who is experiencing the thought and beginning to shiver. Neither 'shiver' nor 'thought' can be the subject because they are not performing the action, they are objects related to the actions of the subject, 'Sean.' The verb in this sentence is 'began,' which denotes an action that Sean started doing. If there were an adverbial phrase, it would provide more information about where, when, why, or how the action took place, but in this case, 'at the thought of going back outdoors' serves as a prepositional phrase, not an adverbial.