This is the 538th most frequent Croatian word.
tvrde
"Tvrdé" in English means "hard" or "firm."
The word "tvrde" is used as a verb which means "claim" in the third person plural present tense.
Oni tvrde da su pronašli izgubljeni grad.
They claim that they have found the lost city.
Here, "tvrde" is the feminine plural form of the adjective "tvrd", which means "hard" or "solid".
Zidovi ove kuće su jako tvrde.
The walls of this house are very hard.
In this example, "tvrdnje" is a noun meaning "claims", derived from the verb "tvrditi".