Answer and explanation:
We can label the sentence in the following manner:
Pavarotti was a great tenor.
S V PN
A subject of a sentence is that sentence's topic, what it is about. The sentence above is about Pavarotti. Thus, Pavarotti is the subject.
"Was" is the past form of the verb "be". "Be" is called a linking verb. Its function is to connect the subject to the information about it (predicate) without conveying an action.
"A great tenor" is a predicate noun. It is a noun phrase whose function is to provide information about the subject. The most important part of this noun phrase is the noun "tenor". Let's be careful with the adjective "great". Its presence here does not make this a predicate adjective. "Great" is not providing information directly about the subject, but it's rather modifying the noun "tenor".