*Je mag
relative pronouns nooit weglaten wanneer er een
werkwoord achter staat.
I told you about the woman who lives next door.
The apple which is lying on the table is not very pleasant.
*Je mag who, which of that wel weglaten wanneer er geen werkwoord achter staat
The apple (which) George lay on the table is very nice.
The car (which) the bankrobbers escaped in was green.