这句话的主句是The English play was a great success.句中play后有一个which引导的定语从句.which前的介词in表示which在定语从句中作介词的宾语.即my students acted in the play at the New Year's party,play 在定语从句中由which来指代.
希望你明白这个句子,也明白关于定语从句引导词的其它情况.