中学生英语读写1.有的介词短语可用来表示时间，这种介词短语实质上起着状语从句的作用，它可以取代一个表示时间的状语从句。例如：At the age of twelve(=When he was twelve)，Edi-son began selling candy，sandwiches and newspapers on a train.(JBVI，P.66)／During the Middle school students’ English reading and writing 1. Some prepositional phrases can be used to represent time. This prepositional phrase essentially acts as an adverbial clause, which can replace an adverbial clause that represents time. For example: At the age of twelve (=When he was twelve), Edi-son began selling candy, sandwiches and newspapers on a train. (JBVI, P.66)/During the