"Tohle slovo znali už v patnáctém století."

Translation: They already knew this word in the fifteenth century.

[MikhailDidenko]

This word was already known in the fifteenth century - this variant should be considered correct as well

[VladaFu]

That is "Toto slovo bylo známé už v patnáctém století.".

Keep the verb in the active voice.

[MikhailDidenko]

It's passive here, this is the linguistics' ABC

[VladaFu]

Yes and it is passive and that is wrong. Keep it active as in the original.

[endless_sleeper]

It should not. :)