Learn Italian: Abbiamo visto la basilica di San Pietro, che è un gran capolavoro. - We have seen the basilica of Saint Peter, which is a great masterpiece.

The word santo (saint) drops its final syllable (becomes san) when it comes directly before a masculine name that starts with a consonant. There are a few exceptions, like names starting with s plus another consonant or z, but since Pietro starts with a single consonant, San is the correct form.

Why do we use gran instead of grande before capolavoro?

Just like santo, the adjective grande (great or big) drops its final syllable (becomes gran) when placed directly before a singular masculine noun starting with a consonant. Because capolavoro starts with the consonant c, we use gran to make the phrase flow better.

Could I say un capolavoro grande instead?

You could, but the meaning shifts slightly. When grande comes after the noun, it usually means physically large. When placed before the noun and shortened to gran, it means great in a figurative sense, emphasizing its high quality or importance.

What kind of word is capolavoro?

It is a compound noun made of capo (head or chief) and lavoro (work). Putting them together gives you the chief work, which translates to masterpiece in English.