it never occurred to humans until the mid-1990s( ) some animals can be cloned．请问这里为什么填that?when 那这里填that是怎么解释的呢？前辈，还是不懂呀
问题描述：
it never occurred to humans until the mid-1990s( ) some animals can be cloned．
请问这里为什么填that?when
那这里填that是怎么解释的呢？
前辈，还是不懂呀
答
这里的that所引导的从句some animals can be cloned是指前面的'it“，你如果用when就没法解释了。你可以把 it换成some animals can be cloned就容易理解了。
答
不可以天when
因为这里是It为形式主语
指代后面的some animals can be cloned这件事
that引导主语从句
正常语序为That some animals can be cloned never occurred to humans until the mid-1990s
that引导主语从句不懂吗 还是哪里