Learn Italian: Regoli pure il sedile. - Go ahead and adjust the seat.

By itself, Regoli il sedile is a direct command meaning "Adjust the seat." Adding pure right after the verb softens it into a polite invitation or permission, giving it the feeling of "Go right ahead and adjust the seat" or "Feel free to adjust the seat."

Why does the verb regolare end in "-i" here (regoli)?

This is the formal imperative, used when addressing someone politely as Lei. For -are verbs like regolare, the formal command takes an -i ending. If you were speaking informally to a friend, you would use the tu form, which uses the -a ending: Regola pure il sedile.

Does sedile mean the same thing as sedia?

Not quite! A sedia is a standard piece of furniture, like a dining chair. A sedile is a seat that is built into a vehicle or structure, such as a car seat, an airplane seat, or a seat on a train.

If I wanted to say "Go ahead and adjust it" instead of "the seat", how would the sentence change?

Because this is a formal command, the direct object pronoun lo (it) must go before the verb: Lo regoli pure. If it were an informal command, the pronoun would attach directly to the end of the verb instead: Regolalo pure.