YOU SAID:

As I was walking up the stairs I met a man who wasn't there

INTO JAPANESE

私が階段を歩いていたとき、私はそこにいない人に会った

BACK INTO ENGLISH

When I was walking down the stairs, I met a person not there

INTO JAPANESE

私が階段を歩いていたとき、私はそこにいない人に会った

BACK INTO ENGLISH

When I was walking down the stairs, I met a person not there