A linking verb connects a subject to a subject complement which identifies or describes the subject, as in the following sentences:
The play is Waiting for Godot.
In this sentence, the linking verb "is" links the noun phrase "the play" to the identifying phrase "Waiting for Godot," which is called a subject complement.
Some of us thought that the play was very good.
In this sentence, the verb "was" links the subject complement "very good" to subject "the play."
Others thought it became tedious after the first fifteen minutes.
In this sentence, the linking verb "became" links the subject "it" to the subject complement "tedious." The phrase "after the first fifteen minutes" functions as an adverb modifying the clause "it became tedious."
The cast appears disorganised and confused; perhaps Beckett intended this.
Here "appears" is functioning as a linking verb that connects the subject "the cast" to its subject complement "disorganised and confused."
The play seems absurd to me.
The subject "the play" is joined to its subject complement "absurd" by the linking verb "seems."
Linking verbs are either verbs of sensation ("feel," "look," "smell," "sound," "taste") or verbs of existence ("act," "appear," "be," "become," "continue," "grow," "prove," "remain," "seem," "sit," "strand," "turn").
Many linking verbs (with the significant exception of "be") can also be used as transitive or intransitive verbs. In the following pairs of sentences, the first sentence uses the highlighted verb as a linking verb and the second uses the same verb as either a transitive or an intransitive verb:
Griffin insists that the water in Winnipeg tastes terrible.
In this sentence, the adjective "terrible" is a subject complement that describes a quality of the water.
I tasted the soup before adding more salt.
Here the noun phrase "the soup" identifies what "I tasted." "The soup" is the direct object of the verb "tasted."
My neighbour's singing voice sounds very squeaky despite several hours of daily practice.
In this example, the phrase "very squeaky" is a subject complement that describes or identities the nature of the "singing voice."
Upon the approach of the enemy troops, the gate-keeper sounded his horn.
Here the verb "sounded" takes a direct object, the noun phrase "his horn."
Cynthia feels queasy whenever she listens to banjo music.
In this sentence, the adjective "queasy" is a subject complement that describes Cynthia.
The customer carefully feels the fabric of the coat.
Here the noun phrase "the fabric of the coat" is the direct object of the verb "feels" and identifies what the customer feels.