YOU SAID:

i'm in love with a guy who's last name i don't know

INTO JAPANESE

私は知らない姓の男に恋をしています

BACK INTO ENGLISH

I'm in love with a man with an unknown surname

INTO JAPANESE

姓がわからない男に恋をしています

BACK INTO ENGLISH

I'm in love with a man who doesn't know his last name

INTO JAPANESE

私は彼の姓を知らない男に恋をしています

BACK INTO ENGLISH

I am in love with a man who does not know his surname

INTO JAPANESE

私は彼の姓を知らない男に恋をしています

BACK INTO ENGLISH

I am in love with a man who does not know his surname