Learn Italian: Hai per caso visto le mie chiavi? - Have you by any chance seen my keys?

Per caso means "by chance" or "by any chance". Just like in English, it is very common in Italian to place short adverbs or adverbial phrases directly between the auxiliary helping verb (hai) and the past participle (visto).

Is visto an irregular verb form?

Yes! It is the past participle of vedere (to see). Regular verbs ending in -ere usually take an -uto ending in the passato prossimo (past tense), but vedere uses the irregular form visto.

Why do we say le mie chiavi instead of just mie chiavi?

In Italian, you almost always must use a definite article (like il, la, i, or le) right before a possessive word. Because "keys" is feminine plural, it becomes le mie chiavi (literally "the my keys").

If chiavi ends in "-i", why does it use the feminine words le and mie?

The singular word for key is la chiave. It belongs to a specific group of Italian nouns that end in -e in the singular and can be either masculine or feminine. When making them plural, they always change to end in -i, regardless of their gender. So the feminine la chiave becomes le chiavi.