Hard

difícil

(requiring a lot of effort to do or understand)

difícil

(demanding a lot of effort to endure)

difícil

(severe)

duro

(resistant to pressure)

duro

(demanding a lot of effort to endure)

duro

(of water, high in dissolved calcium compounds)

forte

(of drink: strong)

inquestionável

(unquestionable)

rigoroso

(demanding a lot of effort to endure)

severo

(severe)

árduo

(demanding a lot of effort to endure)

bem

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