The box was so heavy that I couldn't lift it .中为什么用couldn't 而不用can't
问题描述：
The box was so heavy that I couldn't lift it .中为什么用couldn't 而不用can't
答
因为前面was是过去时，统一时态后面就用couldn't
答
时态要保持一致,前面用是的was,所以这里用couldn't.