1.Of many thousand spare parts does not come up to the standard but one.
在成千上万个备件中,只发现一个不合格。
把句子的语序变一下:But (only) one of many thousand spare parts does not come up to the standard.
这样就好理解了。
2.That which is good cannot be done too soon.
好事做得越快越好。
这是英语中的understatement修辞手法,即否定其反面来达到肯定的目的。这句可以理解为:好事做得多早都不过份。类似的还有:I cannot thank you enough. 谢你谢多少都不够(非常感谢你)