YOU SAID:
How did you know I'm learning Japanese???
INTO JAPANESE
私は日本語を勉強していることをどうやって知った？
BACK INTO ENGLISH
How did I know that I am studying Japanese?
INTO JAPANESE
私は日本語を勉強していることをどのように知っていましたか？
BACK INTO ENGLISH
How did I know that I am studying Japanese?
You've done this before, haven't you.
