'who, which, that' nie pominiesz w uzyciu w przypadku, kiedy wyrazaja one dopelnienie:
The man who/that is talking to your mother is my English teacher. (ktory rozmawia z)
The man (who/that) my mother is talking to is my English teacher. (z ktorym moja mama rozmawia)
The computer which/that is being used by Monica belongs to my boss. (ktory jest uzywany)
The computer (which/that) Monica is using belongs to my boss. (ktorego Monika uzywa)
Poza tym, nie wolno pomijac 'who' oraz 'which' w zdaniach typu non-defining:
Robert Kubica, who is one of the youngest F1 drivers, has won another race in his career.
His racing car, which is a BMW, is thought to be one of the slowest F1 cars in this season.