The subject is the actor, what the sentence is about.
Professor Mongoose loved Melody Moonbeam.
Melody Moonbeam did not love the professor.
The verb shows the action performed by the subject or the state of being which the subject is in.
(If any purists object to the sentence above, which ends with a preposition, the Grammar Doctor invites you to look at prepositions.)
Aristotle Mongoose gave chocolates to Melody Moonbeam.
Melody would not eat chocolates from Mongoose.
Aristotle Mongoose pined over the beautiful Melody Moonbeam.
He could envision the sunlight in her hair at the demolition derby.
Other verbs show a state of being.
Mongoose was heart sick.
Melody Moonbeam was a cruel bitch.
Aristotle Mongoose loved chocolate.
He directed the Chocolate Lab at Hedgehog College.
State of being verbs always require an ending, something that comes after the verb. The ending always relates to the subject of the sentence.
One kind of ending that comes after a state of being verb is a predicate noun. A predicate noun gives another name or designation to the subject.
Melody Moonbeam was a designing woman.
Aristotle Mongoose could be an overbearing ass.
Another type of ending that comes after a state of being verb is a predicate adjective. A predicate adjective comes after the verb, but it describes the subject of the sentence.
Mongoose was foolish
Melody Moonbeam was cruel.
State of being verbs are also called linking verbs because they link the subject to end of the sentence.
Everything in a sentence is a subject, a verb, a sentence ending, or a modifier. Modifiers describe other sentence elements.