YOU SAID:

I wrote them down on a postcard

INTO JAPANESE

私はそれらを葉書に書いた

BACK INTO ENGLISH

I wrote them in postcard

INTO JAPANESE

私ははがきでそれらを書いた

BACK INTO ENGLISH

I wrote them with postcards

INTO JAPANESE

私はポストカードでそれらを書いた

BACK INTO ENGLISH

I wrote them with a postcard

INTO JAPANESE

私ははがきでそれらを書いた

BACK INTO ENGLISH

I wrote them with postcards

INTO JAPANESE

私はポストカードでそれらを書いた

BACK INTO ENGLISH

I wrote them with a postcard