YOU SAID:

the dog was running; i chased after it

INTO JAPANESE

犬は走っていた；私はそれを追いかけた

BACK INTO ENGLISH

the dog was running; I chased it

INTO JAPANESE

犬は走っていた。私はそれを追いかけた

BACK INTO ENGLISH

The dog was running. I went after it.

INTO JAPANESE

その犬は走っていた。私はそれを追いかけた。

BACK INTO ENGLISH

The dog was running. I went after it.