My friend came to my home last night .But it happened (1)that I was about to go out.(1)为什么用that,不用when
问题描述：
My friend came to my home last night .But it happened (1)that I was about to go out.
(1)为什么用that,不用when
答
（1）中的about to go out 就是有问题，应该成about going out。
可以用when啊，从句的导语不是固定的
答
不是"发生在我出去的时候".而是"不巧的是,我出去了."懂?...
答
it happened that 固定用法,表示正巧.的意思