1. Forms of the verb to be (am, are, is, was, were, etc.)
2. Verbs related to the five senses (look, sound, smell, feel, taste)
3. Verbs that reflect a state of being (appear, seem, become, grow, turn, prove, remain).
A linking verb connects a subject and its complement. There are two kinds of complements: noun complements or an adjective complements.
Those people are all professors. NOUN COMPLEMENT
Those professors are brilliant. ADJECTIVE COMPLEMENT
This room smells bad. ADJECTIVE COMPLEMENT
I feel great. ADJECTIVE COMPLEMENT
Verbs that reflect a change in state of being are sometimes called resulting copulas. They link a subject to a predicate adjective:
His face turned purple.
She became older.
The milk has gone sour.
The crowd grew ugly.