YOU SAID:

I once saw this girl at a party

INTO JAPANESE

私はかつてこの女の子を党で見た

BACK INTO ENGLISH

I used to see this girl in a party

INTO JAPANESE

私はこの女の子をパーティーで見た

BACK INTO ENGLISH

I saw this girl at the party

INTO JAPANESE

私はパーティーでこの女の子を見た

BACK INTO ENGLISH

I saw this girl at the party