Learn Hardcore Russian: Это книга.

In Russian, the present tense of the verb to be is typically omitted. The sentence directly links the subject and the predicate without an explicit verb. Thus, Это книга. effectively means "It is a book."

What is the function of the word это in the sentence?

Это serves as a demonstrative pronoun that functions as the linking element in identification sentences. It introduces or identifies the object in question and can be understood as "this" or "it" depending on the context.

Why is книга in the nominative case in Это книга.?

Since there is no explicit linking verb and the sentence is an identification statement, книга remains in the nominative case as a predicate nominative. This usage is standard in Russian for sentences that state a fact or define something.

How does Это книга. differ from Эта книга.?

Although they look similar, Это книга. is a complete sentence meaning "This is a book," with это serving as a neuter demonstrative pronoun in the predicate. In contrast, эта книга is a noun phrase meaning "this book," where эта is the feminine demonstrative adjective modifying книга. The choice depends on whether you are making an identification or simply referring to an object.

Can adjectives be added to this sentence, and if so, how should they agree?

Yes, adjectives can be added to provide more detail. For instance, to say "This is an interesting book," you would say Это интересная книга. Here, the adjective интересная agrees with книга in gender (feminine) and number, while это remains unchanged as the neuter linking element in the sentence.