Under what circumstances should prepositions be added to the antecedents of attributive clauses?

Under what circumstances should prepositions be added to the antecedents of attributive clauses?

First of all, you can't distinguish the antecedent from the guide word. For example, in this is the book which I am looking for, book is the antecedent and which is the guide word. In he is Mr. Robertson who comes from England, Mr. Robertson is the antecedent and who is the guide word