YOU SAID:

During the horseback ride, I stole the knife from him.

INTO JAPANESE

乗馬中に、私は彼からナイフを盗んだ。

BACK INTO ENGLISH

While riding, I stole a knife from him.

INTO JAPANESE

乗っている間、私は彼からナイフを盗んだ。

BACK INTO ENGLISH

While riding, I stole a knife from him.