YOU SAID:
I don't know about any letter
INTO JAPANESE
手紙は知りません
BACK INTO ENGLISH
I don't know the letter
INTO JAPANESE
手紙がわかりません
BACK INTO ENGLISH
I don't understand the letter
INTO JAPANESE
手紙がわかりません
BACK INTO ENGLISH
I don't understand the letter
Come on, you can do better than that.