Be can have many different forms depending on its subject and on its tense:
Be can be used in the following ways:
as an auxiliary verb in progressive verb tenses or in passive forms (followed by a present participle or a past participle): The train is leaving.A window was broken.
as a verb (followed by an infinitive with ‘to’): She is to marry Lord Stanhope.All books are to be returned by Friday.The party was to be a surprise.
as a linking verb (followed by an adjective or noun complement): Dad was ill.His wife is a doctor. (followed by an adverb or preposition): The children are in bed.
Questions and negatives that are formed without ‘do’: Are you busy?The book was not expensive.
Be is often used in question tags: You weren’t listening, were you?It’s cold, isn’t it?