YOU SAID:
Mary had a dog who died yesterday
INTO JAPANESE
メアリーは昨日死亡した犬を飼っていました
BACK INTO ENGLISH
Mary had a dog that died yesterday
INTO JAPANESE
メアリーは昨日死んだ犬を飼っていた
BACK INTO ENGLISH
Mary had a dog that died yesterday
That didn't even make that much sense in English.