Hard

ciężki

(demanding a lot of effort to endure)

mocny

(of drink: strong)

niepodważalny

(unquestionable)

surowy

(severe)

trudny

(requiring a lot of effort to do or understand)

twarda

(of water, high in dissolved calcium compounds)

twardy

(resistant to pressure)

ciężko

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