We're getting a bit into the realm of nuances and feelings with this question :). I think that the first suggested meaning by you: "I don't exactly understand him" is not a correct translation. It sounds to me more like that you either do not understand what he says exactly (like literally understand his words or language) or that you don't understand his behaviour that much... like "I don't exactly like this" doesn't mean that you totally hate it... it's just that you don't kinda like it... In the second translation you insert this "even" which is lacking from the Czech version... In the third one you use this "actually", which is also not in the Czech sentence. If I were to choose and extra word in English that would capture the essence of the Czech sentence, it would be "absolutely" ... "I absolutely don't understand him" which means, and that's important to note, that you do not understand his behavior/his decision/.... The Czech "nechápu" has nothing to do with "understanding" a language or spoken word, it means "understanding a person/an idea/math/..." So I guess, after all, it's up to you. All of the three sentences you mentioned mean roughly the same thing, adding or lacking a little bit from the official version, but definitely bearing 90% of the necessary meaning :).