句子中的doing 和 reading 是分词.Students can find everything they need to look up when doing research paper or reading for fun这个句子也可以这样写——Students can find everything they need to look up when （they are）doing research paper or（ when they are） reading for fun.这就是分词短语作状语时和状语从句之间的关系,是使用进行时态的状语从句的简化.