Another "who/whom" Conundrum, 2 (see yesterday first)

       "It's (he/him) (who/whom) you can't understand."

       This is a difficult one, but “he” and “whom” are correct. 

       Any form of the verb “to be” does not take an object following it, only a subject.  For example,  one should say, “It is I” not “It is me.”   “He” is a subject, “him” is an object; hence “It’s he” is correct.

       “(Who/whom) you can’t understand” is a separate  clause in which the word in question is the object of the verb “understand.”  To check which  sounds right, change the word order around and substitute “he” and “him” for “who” and “whom":  “You can’t understand he” doesn't sound right; “him” does.  “Whom” is an object like "him"; so the correct choice is “whom you can’t understand.”

       "It's he whom you can't understand."