What is the subject of the following sentence?: The teacher covered several subjects today in history class.
The complete subject (everything before the predicate) is "the teacher." The simple subject is "teacher."
The subject is who or what the sentence is about. The subject is either a noun or pronoun. It receives/does the action in the sentence (if the simple predicate is an action verb).
The main focus of the sentence is the teacher, not the subjects or the history class.