Learn Hardcore Italian: Il libro è aperto.

In Italian, il is the standard masculine singular article used before most consonants (like L in libro). Lo appears instead before words starting with S + consonant (like lo studente), Z (like lo zaino), GN, PS, and a few other specific cases.

Why is è used here to mean is?

È is the third-person singular form of the verb essere (to be). It simply means is. Note that c’è (there is) and ha (he/she has) are different verbs or constructions and change the meaning.

Why does aperto end in -o and not -a or -i?

Because il libro is masculine and singular. Italian adjectives and participles must match the gender and number of the noun. So, for a masculine singular noun, you end in -o (e.g., aperto). For a feminine singular noun (like la porta), you would say aperta, and if the subject were plural, you’d make it aperti (masculine) or aperte (feminine).

Is aperto functioning more like an adjective or a past participle here?

It is technically the past participle of the verb aprire (to open), but in this context, it works like an adjective describing the state of the book (the book is open). This dual function is common with many past participles in Italian.