Question

求助英语高手

When it is struck, a tuning fork produces an almost pure tone, retaining its pitch over a long period of time.这句为什么用retaining? The mechanism by which brain cells store memories is not clearly understood.为什么在which前有个by？

Answer 1

retaining:现在分词作伴随状语，表示这个动作与主句中produce这个动作同时发生，
可以改成：and retains its pitch over a long period of time.
mechanism：在这里是“方法”的意思，等于means，
by the mechanism=by the means，by表示通过。。。。（某种途径）
即： brain cells store memories by some（某种） mechanism （途径）

Answer 2

When it is struck, a tuning fork produces an almost pure tone, retaining its pitch over a long period of time当它被敲击，一个有声的叉子发出纯正的音，保持纯正的音很长一段时间。
retaining前省略了主语和系动词it is ，避免重复。
The mechanism by which brain cells store memories is not clearly understood这种通过大脑细胞来储存信息的方式还没有被完全理解。
这是一个非限定性定语从句；介词+which，这里是by which

Answer 3

音叉被敲击的时候，会发出接近纯正音调的声音，这声音会保持它的音高很长一段时间。
根据上述句子翻译，可以看出retianing的主语是“an almost pure tone”，这里-ing形式是现在分词做后置定语，可以改写成which retains its pitch over a long period of time.

The mechanism by which brain cells store memories is not clearly understood.
人们对脑细胞通过什么样的机理储存记忆还没有明确的研究成果。如果用英语表述可以是：Brain cells store memories by certain mechanism. The mechanism is not clearly understood.
如果没有by, 句子就成为：The mechanism which brain cells store memories is not clearly understood. 人们脑细胞储存记忆的机理还没有明确的研究成果。这里的“mechanism”成为了脑细胞储存记忆的地方，用英语说就是：Brian cells store memories in certain mechanism. The mechanism is not clearly understood. 不过按照这样的意思，那which前还要加一个介词 “in”，成为：The mechanism in which brain cells store memories is not clearly understood. 或者把which换成where，成为：The mechanism where brain cells store memories is not clearly understood. 从以上的解释中，可以看出在常规下，我们的理解是脑细胞是通过某种机理来储存记忆的，而这种机理我们还不清楚是如何工作的。