|I don't know.
|I don't understand.
|Do you understand?
Wakari masu ka?
|I don't know how to write it.
Kaki-kata ga wakari masen.
kaki-kata: how to write
|I don't know what this character means.
Kono ji no imi ga wakari masen.
ji: character (a letter, mark, or sign used in writing, printing, or computing)
|I don't know how to say in Japanese.
Nihon-go de dou iu noka wakari masen.
nihon-go: Japanese language