'Forever yours' is an English equivalent of 'Per sempre il vostro'. This is what's said by an eternally devoted or loving male. An eternally devoted or loving female says 'Per sempre la vostra'.
"The world is mine" is an English equivalent of "Il mondo è mio." Specifically, the masculine singular definite article "il" means "the." The masculine noun "mondo" means "world." The verb "è"...