In 1858,a French engineer arrived in England with a plan for a twenty-one-mile tunnel under the English Channel. The tun
问题描述：
In 1858,a French engineer arrived in England with a plan for a twenty-one-mile tunnel under the English Channel. The tunnel would be well-ventilated if tall chimneys were built above sea level.后面一句是虚拟吗?与过去事实相反,应该是用完成时.为什么这里是用过去时.
这个tunnel将会有良好的通风，如果高高的chimney建在海面上
答
第二句话是条件状语从句,条件状语从句的结构是主句将来时,从句一般现在时.现在这件事情是发生在过去的,理所当然用过去时对吧?所以时态各倒退一个,主句用过去将来,从句用一般过去.