Hard

arduo

(demanding a lot of effort to endure)

difficile

(requiring a lot of effort to do or understand)

difficile

(severe)

dura

(of water, high in dissolved calcium compounds)

duro

(resistant to pressure)

duro

(requiring a lot of effort to do or understand)

forte

(of drink: strong)

indiscutibile

(unquestionable)

intenso

(requiring a lot of effort to do or understand)

severo

(severe)

sodo

(with much force or effort (related to act of think))