. The verb to be can serve either as a main verb or as an auxiliary verb. When it acts as a main verb, it typically couples a grammatical subject with an adjective (predicate adjective), as in John is big or The sunset is beautiful. Or it couples a grammatical subject with another noun (predicate noun), as in Mary is president or War is hell.Or it might couple a grammatical subject with a phrase, as inThe book is on the tableorHe is from the South.