YOU SAID:

I've seen aliens on television before

INTO JAPANESE

私は前にテレビで宇宙人を見た

BACK INTO ENGLISH

I saw an alien before on television

INTO JAPANESE

私はテレビで前に外人を見た

BACK INTO ENGLISH

I saw a foreigner in front of me on TV

INTO JAPANESE

私はテレビで私の前に外国人を見た

BACK INTO ENGLISH

I saw a foreigner in front of me on TV