Hard

fest

(resistant to pressure)

fix

(unquestionable)

hart

(resistant to pressure)

hart

(requiring a lot of effort to do or understand)

hart

(severe)

hart

(of water, high in dissolved calcium compounds)

herausfordernd

(requiring a lot of effort to do or understand)

schwer

(requiring a lot of effort to do or understand)

schwer

(demanding a lot of effort to endure)

schwierig

(requiring a lot of effort to do or understand)

schwierig

(demanding a lot of effort to endure)

stark

(of drink: strong)