Learn Italian: Questo è un libro nuovo.

Questo means "this" and is used here because the noun it refers to, libro (book), is masculine. If we were talking about a feminine word like mela (apple), we would use Questa, as in Questa è una mela.

Is Questo acting as an adjective here?

No, in this sentence Questo is acting as a pronoun, which means it stands alone as the subject to mean "This is". If it were an adjective, it would be placed right next to the noun, like in Questo libro è nuovo (This book is new).

Why is it libro nuovo instead of nuovo libro?

In Italian, descriptive adjectives like nuovo (new) usually come right after the noun they describe. So, instead of saying "a new book", you say "a book new" (un libro nuovo).

Why is the article un instead of uno?

Un is the masculine word for "a" or "an" used before words that start with most consonants, like libro. You only use uno before masculine words starting with specific tricky letters, like an "s" followed by a consonant (uno studente) or a "z" (uno zaino).