"This is a book" what is the role of "book"? [closed]

What is book in this sentence ?

• This is a book.

Is that an Object?

What are the nouns that come in the sentences with to be's?

Like my friend in He is my friend?

Solution 1:

This is a book.

I think you want to ask about the noun phrase a book. The word book is the Head of the noun phrase a book. The phrase a book however, is a Complement of the verb be.

Specifically, it is a Predicative Complement. This type of Complement gives us a description of the Subject or Object of a verb. They are usually noun phrases, adjective phrases or preposition phrases:

Subject oriented Predicative Complements:

• He is happy
• He is a tyrant
• He is on fire

Object Oriented Predicative Complements:

• We elected her treasurer
• This made her curious
• She left him in a spin

Notice that when a Complement is an Object, it usually introduces a new entity into the sentence. Predicative Complements describe one of the entities already in the sentence. They usually don't introduce a new one.

Objects can normally become the Subjects of passivised sentences. Predicative Complements can't:

• *Treasurer was elected her by us.
• *Book is been this.

It's probably worth pointing out, however, that there is no restriction on Predicative Complements occurring in passive voice sentences per se. The main point is that they can't be the Subject of passivised sentences. The following is therefore perfectly grammatical:

• She was elected treasurer by us.

In this example it's the Direct Object her/she which is the subject of the sentence. The Predicative Complement still appears as a Complement of the verb elected.

Solution 2:

The auxiliary verb be is used for the Passive construction, for the Progressive construction, in any number of idioms, and also for any predicate that is not a verb. Since verbs are always predicates, we don't have to call them "predicate verbs"; but since nouns and adjectives are often not predicates, it is customary to call them Predicate Nouns and Predicate Adjectives when they are used as predicates. Both predicate nouns and predicate adjectives require an auxiliary be.

In the example sentences, book and friend are both predicate nouns, and that's where the is comes from; it's a required auxiliary verb for a predicate noun. Note that predicate nouns are really noun phrases -- they can have articles, quantifiers, possessives, adjectives, and so on, just like any NP; in

• That man [was [the most ridiculous parody of a public servant that I've ever seen]].

the verb phrase is the outer bracket, and the predicate noun is the inner, for instance.

Executive summary: Use auxiliary be for Predicate Adjectives

• I am firm.
• You are stubborn.

and for Predicate Nouns

• He is a pig-headed fool.