[الاهلاوى 2007]

µ A teacher is a person. A teacher works at a school.(who)
è A teacher is a person who works at a school.
µ I know Ali .He is a good footballer. (who)
è - I know Ali who is a good footballer
µ Ahmad is naughty .I love him. (who)
è Ahmad is naughty who love him
µ I met the actor. You told me about him. (who)
è I met the actor who You told me