1.A "sliver democracy" that spends far more on the the elderly than on eduction and child care.2.The key is to try and avoid a debate over whether your loved one is sick.3.Western culture's continuing development then led to the Scientific Revolution,the Enlightenment,the American Revolution, the Industrial Revolution and to what is considered today as modern civilization.请教老师，为何有时and后面有对应的介词，有时又没？？

网友这个问题，不够明确。

1.A sliver democracy that spends far more on the the elderly than on eduction and child care.

——on eduction and child care：这里，在child care 之前不可以再加on，那就有重复的错误。一个on 管着两个名词，这叫“介词共享”。

2.The key is to try and avoid a debate over whether your loved one is sick.

——try and avoid，都没介词啊。

3.Western cultures continuing development then led to the Scientific Revolution,the Enlightenment,the American Revolution, the Industrial Revolution and to what is considered today as modern civilization.

——第二个 to 需要有，如此做是为了明确告诉读者，前面省去了led，不然的话会导致误读。