ztvrdnout znamená v angličtina

firm [firmed, firming, firms]▼(to become firm)
verb
[UK: fɜːm] [US: ˈfɝːm]

stiffen [stiffened, stiffening, stiffens]▼(to become stiff)
verb
[UK: ˈstɪf.n̩] [US: ˈstɪf.n̩]

toughen [toughened, toughening, toughens]▼(to become tougher)
verb
[UK: ˈtʌf.n̩] [US: ˈtʌf.n̩]