YOU SAID:

I cant belive it started raining cats

INTO JAPANESE

私はそれが猫を雨が降り始めた信じられない

BACK INTO ENGLISH

I can't believe it started raining cats

INTO JAPANESE

猫に雨が降り始めたなんて信じられない

BACK INTO ENGLISH

I can't believe the cat has begun to rain

INTO JAPANESE

猫が雨を降り始めたなんて信じられない

BACK INTO ENGLISH

I can't believe the cat has begun to rain