状语从句和强调句区别

问题描述:

状语从句和强调句区别
-Did Jack come back early last night?
-Yes.It was not yet eight o'clock _____ he arrived home.
答案为when 请问为什么不能是that 强调句?
答案解析说的是强调句强调时间状语要是 It was eight o'clock that he arrived home last night
不太明白

去掉It is that 如果还成句就填THAT
如果题目改成 Did Jack come back early last night?
-Yes.It was not yet at eight o'clock _____ he arrived home.
则填THAT
原题少了AT