YOU SAID:
I cannot understand what he is thinking
INTO JAPANESE
彼が何を考えているのか理解できない
BACK INTO ENGLISH
I can't understand what he's thinking
INTO JAPANESE
彼の考えが理解できない
BACK INTO ENGLISH
I can't understand his idea
INTO JAPANESE
彼の考えが理解できない
BACK INTO ENGLISH
I can't understand his idea
That didn't even make that much sense in English.