YOU SAID:
on my eighteenth birthday i gave her a car
INTO JAPANESE
18歳の誕生日に彼女に車をあげた
BACK INTO ENGLISH
I gave her a car for her 18th birthday
INTO JAPANESE
私は彼女の18歳の誕生日に彼女に車をあげました
BACK INTO ENGLISH
I gave her a car for her 18th birthday
That didn't even make that much sense in English.