YOU SAID:

when i was at the party, it was raining cats and dogs

INTO JAPANESE

私がパーティーにいたとき、それは猫と犬を雨が降っていた

BACK INTO ENGLISH

When I was at the party, it was raining cats and dogs.

INTO JAPANESE

私がパーティーにいたとき、それは猫と犬を雨が降っていました。

BACK INTO ENGLISH

When I was at the party, it was raining cats and dogs.