Bob, who I know very well, is going out with Mary.
"who" is necessary
"who" is not necessary
I cannot remember the hotel that we stayed at.
"that" is necessary
"that" is not necessary
I cannot forget the song which they played last night.
"which" is necessary
"which" is not necessary
Lucy, who I haven't seen for ages, phoned me last night.
"who" is not necessary
"who" is necessary
