YOU SAID:
I swear she told me that she was 18
INTO JAPANESE
彼女は18歳だと言ったと誓います
BACK INTO ENGLISH
I swear she said she was 18
INTO JAPANESE
私は彼女が18歳だと言ったと誓います
BACK INTO ENGLISH
I swear she said she was 18
That didn't even make that much sense in English.