The section on "Relative Clauses" needs Tips and Notes.
Early in the Czech lessons there are Tips and Notes sections that help you to better understand certain principles of the language, like pronunciation, gender, to be singular, etc.
The relative clauses section could really use this. I speak decent Slovak, which is very similar to Czech, but was completely baffled by this section. Tips and Notes should be added to this section with a chart that shows when to use Nimž, Jíž, Němž, Nějž, Nichž, etc.
Without something like this, I think your average learner will have to go through the section 10,000 times in order to really understand this section.
Most of the lessons are fantastic, but this specific section needs a lot of work.
I found this overview (declension table) to be very useful: https://en.wiktionary.org/wiki/jen%C5%BE It's the same like the one you find at http://prirucka.ujc.cas.cz/ , the official language handbook for standard Czech. The only differences (additions) - according to ÚJČ - are the forms of singular genitive masculine (animate and inanimate) and neuter: jehož, jejž /něhož, nějž. Note the helpful advice: "Leading letter j- is changed to n- when the pronoun is preceded by a preposition, e.g. s nímž, k němuž, bez níž."
i just added the tips with a similar table. as usual, the tips length limit prevented me from adding more content. i don't think duo ever expected to have to deal with the more determined students of the more challenging languages. even this length limit may be in jeopardy because of the coming expansion of tips to mobile devices. chop everything down to fit the midget screens.