YOU SAID:
Someone took the fancy necklace that my mother gave me when I turned 18
INTO JAPANESE
私が18歳になったときに母がくれた豪華なネックレスを誰かが取りました
BACK INTO ENGLISH
Someone took the gorgeous necklace my mother gave me when I was 18
INTO JAPANESE
私が18歳のときに母がくれた豪華なネックレスを誰かが取った
BACK INTO ENGLISH
Someone took the gorgeous necklace my mother gave me when I was 18
You've done this before, haven't you.