Hard

жёсткий

žóstkij

(of water, high in dissolved calcium compounds)

кре́пкий

krépkij

(resistant to pressure)

кре́пкий

krépkij

(of drink: strong)

неопровержимый

neoproveržimyj

(unquestionable)

про́чный

próčnyj

(resistant to pressure)

твёрдый

tvjórdyj

(resistant to pressure)

тру́дный

trúdnyj

(requiring a lot of effort to do or understand)

тру́дный

trúdnyj

(demanding a lot of effort to endure)

тру́дный

trúdnyj

(severe)

тяжёлый

tjažólyj

(requiring a lot of effort to do or understand)

тяжёлый

tjažólyj

(demanding a lot of effort to endure)

тяжёлый

tjažólyj

(severe)

си́льно

sílʹno

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