"It is full."
Translation:Het is vol.
I don't know what the exact rule is, but if I'm not mistaken, it goes like: ['het' word] "Een vol bord." "Het volle bord." "Het bord is vol." ['de' word] "Een volle doos." "De volle doos." "De doos is vol." The adjective form depends on wether the noun is a 'de' or 'het' word. As well as wether we are refering to a specific object or not.