"I do not know the fourth one."

Translation: Toho čtvrtého neznám.

[Mamke1]

Why not "Neznam toho ctvrteho"?

[BoneheadBass]

"Neznám toho čtvrtého" is accepted. Perhaps there was something else in your answer that Duo didn’t like. For troubleshooting, it’s best to either (1) use the report button in addition to commenting, or (2) include a screenshot in your comment, so that the team can see your complete answer. (Reporting is easier.)

[Gavin753673]

Why does the adjective come after in this case?

[VladaFu]

It does not. Understand this as if the actual object was skipped "toho čtvrtého (člověka)" "the fourth (man)/one"

[JakubCajthaml]

I went with Toho čtvrté neznám, thinking since I didn't know what one was, it should be neuter. My answer was wrong.

Now, that I read here the implied object is člověka, requiring the masculine. But, could the implied object not also be dítě requiring čtvrté, or holky requiring čtvrtou?

Or, is there some principle of Czech I'm missing?

Edited a few minutes later: Because of the toho!!! That's the principle I'm missing! Yikes!

To čtvrté neznám. Tu čtvrtou neznám.

Or, Ten čtvrtý neznám to complete the set.

[VladaFu]

Yes, toho is masculine, it must be "toho čtvrtého". Neuter is "to čvrté".

[BrandonCas821536]

Please help me to understand why my response is not correct. "Toho čtvrtého jednu neznám." Is "one" understood without actually using jednu?

[Nemesis_NaR]

It is understood without translating the "one". From Czech point of view, it's strange why English actually needs it there (probably because of missing declensions - Czech declensions make it often more clear what exactly I mean).

You can say "Toho čtvrtého muže/člověka neznám", but that has better English translation "I don't know the fourth man/person."

PS: You're mixing masculine and feminine numerals here. "Čtvrtého" is masculine while "jednu" is feminine ending.