The verb BE can be used as a main verb or as an auxiliary.
When used as the main verb, it doubles as the auxiliary and the main verb. This is the only time in the English language when the main verb and the auxiliary verb are one and the same!
The verb BE is used as a main verb in two ways:
with complementsComplements (adjectival phrases and nominal phrases) link BE or other linking verbs with the subject of the clause. Complements are an important function of nouns and adjectives.
with a prepositional phrase
Prepositional phrases indicate time or location.