YOU SAID:
the red fox is running in the leaves
INTO JAPANESE
キツネが葉の中を走っている
BACK INTO ENGLISH
The fox is running through the leaves
INTO JAPANESE
キツネは葉を走っています。
BACK INTO ENGLISH
The fox is running on the leaves.
INTO JAPANESE
キツネは葉の上を走っています。
BACK INTO ENGLISH
The fox runs on the leaves.
INTO JAPANESE
キツネは葉の上を走ります。
BACK INTO ENGLISH
The fox runs on the leaves.
That didn't even make that much sense in English.