translate the following sentence into English. pater servum misit qui hunc librum emeret

Translation:the father sent the slave to by this book.
This is a purpose clause therefor requires the imperfect subjunctive, emeret (he might buy) in the subordinate clause. Because the main verb, misit, is a verb of sending we use a qui rather than an ut to introduce the purpose clause thus triggering the subjunctive verb.
To for the imperfect subjunctive you must take the present active infinitive (emere) then add the correct personal ending (-t).