Translation help...

I need a translation help. (English to Japanese)
"It is so hard to understand him." How can we translate this to Japanese? (Kare wo/ni/ga wakarinikui da ze...) Which one am I gonna use?
And another question:
Does the word "ununderstandable" exist? It is gonna be "Wakararenai" ?

Sankyuu...soshite yoi ichinichi wo ^_^

"It is so hard to understand him." How can we translate this to Japanese?

In textbook Japanese: kare (no koto) wo rikai suru no wa totemo muzukashii (desu).

Does the word "ununderstandable" exist? It is gonna be "Wakararenai" ?

Rikai dekinai/dekimasen
Rikai-funou da/desu

I imagine this person is in 3rd grade & counting 1, 2 ...
And wakariniku is in my dictionary as difficult to understand, so that one would be right. Wakareru means splits - if the ra isn't meant to be there. maybe its an emphasis in some regions to add bits like we do sometimes. It's difficult to follow everything a person says anyway, so keep it simple.

nai on the end means not, so unless its a name at the front of the sentence, then a verb that relates to the previous words, as Japanese is logical, if nothing else.

Hai.

Wakarimashita.

(Japanese use past tenses for now, especially in writing)

kare (no koto) wo rikai suru no wa totemo muzukashii

His item (wo, o is more singular, think about chemistry, Japanese like to order everything, some things are the same but different in number - for more than one item is used)

rikai - understanding (present state) do (suru) with total (totemo) understanding. I'd say this person is confident in his school work or text. But he is a child and doesn't know all the rules.

Sayoonara

Hmmm...Wakarimashita. Doumo arigatou gozaimashita.