Learn Hardcore Italian: Ci sono trenta libri sullo scaffale.

C’è is the contraction of ci + è and is used with singular nouns (“there is”). Here we have trenta libri (30 books), a plural subject. For plural we say ci sono (“there are”), combining ci + sono.

What role does ci play in Ci sono?

In ci sono, ci is an expletive or “dummy” pronoun that doesn’t have a specific meaning on its own—it simply allows the sentence to state existence. In English we don’t translate it directly; we just say “there are.”

Why is there no article before trenta libri?

When you use a number as a determiner (e.g., trenta, venti, cinque), it directly modifies the noun and you drop the indefinite article. Saying trenta libri is equivalent to English “thirty books,” not “the thirty books.”

Does trenta agree in gender or number with libri?

No. Italian cardinal numbers like trenta are invariable: they never change for gender or number. You always say trenta libri (books, masculine) or trenta pagine (pages, feminine).

What is sullo in sullo scaffale?

Sullo is the contraction of the preposition su (“on”) + the masculine definite article lo (used before words starting with sc-, z-, etc.). So sullo scaffale literally means “on the shelf.”

Could I say sulla scaffale or sui scaffali instead?

Sulla scaffale is incorrect because sulla = su + la, and scaffale is masculine, so you need su + lo = sul(lo).
Sui scaffali means “on the shelves” (plural). If you have multiple shelves you’d switch both noun and article to plural: ci sono trenta libri sui scaffali.

Is the word order Ci sono trenta libri sullo scaffale fixed? Could I say Trenta libri ci sono sullo scaffale?

Standard word order is Ci sono [number] [noun] [prepositional phrase]. While Italian’s flexible syntax sometimes allows you to move elements for emphasis (e.g., “Trenta libri ci sono…”), that sounds literary or emphatic. In everyday speech you’d keep the usual order.