为什么不能用can'twithout gravity,we couldn't walk on the ground as freely as we do now变为without gravity,we can't walk on the ground as freely as we do now
问题描述：
为什么不能用can't
without gravity,we couldn't walk on the ground as freely as we do now
变为without gravity,we can't walk on the ground as freely as we do now
答
其实这是虚拟语气.....
Without引导的隐含虚拟语气，从句为一般现在时则变为一般过去式
因为“we walk on the ground"是现在都在做的事
所以can‘t变成couldn't
答
这个句子是if引导的虚拟语气 ，if省略，从句的时态由现在时，变为过去式，
答
这个就是虚拟语气.
没有重力,我们就不可能象我们现在那样在地上走.
其实,重力是存在的.所以,用过去式表示虚拟的.
又如：
If I were you,I would leave.
如果我是你,我就走.
（我不可能是你的：虚拟的）