Subject Complements
A subject complement is the word or phrase that follows a linking verb (like 'to be', 'seem', 'become') and identifies or describes the subject.
Predicate Adjectives
An adjective that describes the subject.
The soup is delicious. (Describes soup) He seems tired. (Describes him)
Predicate Nominatives (Nouns)
A noun or pronoun that renames or identifies the subject.
She became a doctor. (Doctor = She) My neighbor is a musician. (Musician = neighbor)
Why it matters
Because they act as a mirror to the subject, you must use subject pronouns (I, he, she) after linking verbs in formal grammar, not object pronouns (me, him).
Formal: It was I who knocked. (Informal: It was me).