YOU SAID:

I saw someone steal a dog at the park today

INTO JAPANESE

今日誰かが公園で犬を盗むのを見た

BACK INTO ENGLISH

I saw someone steal a dog in the park today.

INTO JAPANESE

今日誰かが公園で犬を盗むのを見た。

BACK INTO ENGLISH

I saw someone stealing a dog in the park today.

INTO JAPANESE

今日誰かが公園で犬を盗んでいるのを見た。

BACK INTO ENGLISH

I saw someone stealing a dog in the park today.