Question

I do not understand.(answer it by changing the voice)

Answer 1

understand IS not done by me

Answer 2

Any sentence can be changed into passive only if there is an object.

Since, The sentence " I do not understand " doesn't has any object it cannot be changed into other voice.

Rather if it would be " I do not understand it" , then the voice change will be as :-

It is not understood to me.

$\huge{\underline{\bf{Hope\:it\:helps}}}$