@peterkotrcka yes, there's a logic. But there are also local variations, so people tend to make mistakes. For example: "siete andati via?" is the correct form. But my grandmother (and many people from her hometown) used to say "siete andato via?" - which is wrong. But it comes from the dialectal, local form, which didn't make any difference between the singular "sei andato" and plural "siete andati" so they used to "translate" a mix.