YOU SAID:

I forgot their house keys

INTO JAPANESE

私は家の鍵を忘れた

BACK INTO ENGLISH

I forgot my house key

INTO JAPANESE

私は自分の家の鍵を忘れた

BACK INTO ENGLISH

I forgot my key at home

INTO JAPANESE

家で鍵を忘れた

BACK INTO ENGLISH

I forgot my key at home.

INTO JAPANESE

私は自宅で鍵を忘れてしまった。

BACK INTO ENGLISH

I forgot my key at home.