Precursor

-- 难句 --

The ox-drawn plow is significant because it represents the first timepeople could use something besides their muscles for motive power, and in thissense the plow was the precursor of the steam engine, the internal combustionengine, the electric generator, and the fission reactor.

-- 结构 --

主句1：Plow is significant.

because it represents the first time 原因状语从句

(when) people could use something besides their muscles for motive power 定语从句修饰time

主句2: The plow was the precursor.

-- 词汇 --

1. motive power 驱动力

2. in this sense 从这个意义上来讲

3. precursor of sth. 某某东西的先驱

4. fission reactor 核裂变反应堆

-- 下期难句 --

As early as 3000B.C.E., Sumerians were carefullystudying and recording the movements of heavenly bodies, their motive againbeing utilitarian: they believed that the will of the gods determined celestialmovements and that knowledge of those movements would enable man to know thedivine will and act accordingly.