What is the difference between rozumim and chapu?
It is good to search first, but I did that for you:
Oh thank you, you shouldn't have done that. I could have searched it on my own ;)
Rozumim je - is not valid?
Rozumím jim. (rozumět + dat) vs. Chápu je. (chápat + acc)
Why is 'ja chápu ně' correct? There is no preposition.
It is not correct. If it was accepted, our settings were ignored. Sometimes Duolingo tries too hard to make its users happy.
Okay then. Thank you.
why not; Mne or me je chapu.
The same as your last question. Já is the subject of the sentence, it must be nominative. It is the same in English. It is "I", not "me".