Copular verbs or linking verbs are intransitive verbs of incomplete predication because they are followed by an obligatory element that tells us something about the subject.
This obligatory element is called ´subjective´ complement or predicative because it refers to the subject.
We can predicate:
*an essence: Sugar is sweet.
*an accident or temporary state: John is tired.
*a class of objects: This is a book.
*identity: That is John.
*a location: He is in the room.
The verb ´to be´ is the most typical copular verb.
John is happy.
The bridge seems safe.
You look pretty tonight.
He turned traitor.
She became a lowyer.
Finite verbs (conjugated):
The door is open. (simple present)
John came yesterday. (simple past)
Non-finite verbs (not conjugated)
To see her is to love her.
I love riding.
*the present participle
I was reading.
*the past participle
I´ve broken my leg.