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?