YOU SAID:

they did surgery on my dad

INTO JAPANESE

彼らは私の父の手術

BACK INTO ENGLISH

They're my dad's surgery

INTO JAPANESE

彼らは私のお父さんの手術

BACK INTO ENGLISH

They are my father's surgery

INTO JAPANESE

彼らは私の父の手術です

BACK INTO ENGLISH

They are my father's surgery