It is ... that 和 It is ... when都是表示强调。显然It is ... when特指“就在。。的那个时候”而it is...that是泛指某个事物。同类型的还有It is ...who "正是某某人。。。"It is ... where“正是在。。。的那个地方”It was that/when 只是It is that/when的过去式。下面来几个例子吧：It was Lisa who helped me with my homework. (正是lisa帮助我完成了作业) 这里也可以用that来代替whoIt was yesterday when I finished my homework.（就在昨天，我完成了作业）这里也可以用that来代替when.It was my homework that gave me a headache.(正是我的作业让我头疼不已)结论就是that是万金油，所有情况通杀。其他的呢就要看具体指代的是人，物，时间还是地点了。
