I run four miles every morning. Which word or words make up the complete subject in this sentence?

I

Explanation: To find the subject of a sentence you need to find the noun being talked about. The noun would not be "four miles" or "every morning" because there is no verb talking about them. "I" on the other hand, is being talked about. The verb "run" is describing what "I" did.

comfortable

explanation:

one type of the adjectives is predicate adjective. a predicate adjective is an adjective which follows a linking verb and modifies or refers back to the main subject of that linking verb.

it usually is used just before the noun it modies.

here in this example:

'penguins seem comfortable in the coolest habitats', comfortable is the predicate adjective which follows the linking verb 'seem' and modifies the noun.