The document defines and provides examples of subject complements and object complements. A subject complement completes the subject of a sentence by providing additional information about it, as in the sentence "Every morning is a gift" where "a gift" is the subject complement. An object complement provides more details about the object of the sentence, such as in "He makes me very sad" where "very sad" is the object complement describing the object "me".

39.  It's a word, clause, or phrase that's needed to complete a given
expression.
 For example, "Every morning is a gift." In this sentence, "every
morning" is the subject, "is" is the linking verb, and "a gift" is the
complement. It completes the idea. Without it, we wouldn't
understand what every morning is.

40.  The subject of a sentence is the person, place, thing, or idea
that the sentence is about.
 Subject complements follow linking verbs only (and not
action verbs).
 Example- Brenna is a gifted singer. (Providing more
information about Brenna)
 Object complements follow linking verbs as well. However,
they provide more information about the object. So, while the
object of the sentence receives the action of the verb, object
complements can beef up, or complement, those objects.
 Example-He makes me very sad. (Providing more
information about the object “me”)