nce4-42课,第一段.An earthquake comes like a thief in the night,w

nce4-42课,第一段.An earthquake comes like a thief in the night,without warning.(as follows)
An earthquake comes like a thief in the night,without warning.It was necessary,therefore,to invented instruments that neither slumbered nor slept.Some devices were quite simple.One,for instance,consisted of rods of various lenghts and thicknesses which would stand up on end like ninepines.When a shock came,it shook the rigid table upon which these stood.If it were gentle,only the more unstable rods fell.If it were severe,they all fell.Thus the rods,by falling,by the direction in which they fell,recorded for the slumbering scientist the strength of a shock that was too weak to waken him,and the direction from which it came.
整段的时态求解释.

An earthquake comes like a thief in the night,without warning.陈述句,一般时态.因为地震一直都是这样,是一般真理.
It was necessary,therefore,to invented instruments that neither slumbered nor slept.过去时态.因为早就应该发明地震仪了.
Some devices were quite simple.One,for instance,consisted of rods of various lenghts and thicknesses which would stand up on end like ninepines.When a shock came,it shook the rigid table upon which these stood.If it were gentle,only the more unstable rods fell.If it were severe,they all fell.Thus the rods,by falling,by the direction in which they fell,recorded for the slumbering scientist the strength of a shock that was too weak to waken him,and the direction from which it came.
过去时态.因为讲的是以前发明的地震仪的工作原理.
再问： 1.If it were gentle,only the more unstable rods fell.If it were severe,they all fell.有虚拟语气吧？ 2.It was necessary,therefore, to invented instruments that neither slumbered nor slept.这里的necessary为什么放therefore前面？ 整篇文章，过去时加着现在时，例如：That sounds a silly suggestion,but that was precisely the idea adoped in some of the early instruments for recording quake waves. 谢谢
再答： 1. If it were gentle,only the more unstable rods fell.If it were severe,they all fell.有虚拟语气 2. Therefore 作为插入语，一般放在主语、动词或者助动词后，两边分别加上逗号。这里插入位置不常见，但是也是允许的。当然放在主语it、动词was后也是可以的，说话习惯因人而异。 3. 整篇文章，讲过去的事情用过去式，讲现在的评论时用现在式（如That sounds a silly suggestion）