When the twins (arrive) their school,they found there was nobody there

问题描述:

When the twins (arrive) their school,they found there was nobody there

直接加d+at 因为该状语从句时间是相继发生,因此两个都用一般过去式。

arrived at
从整个句子来看,要用过去时,而且学校是小地方,要用at不用in