有关介词加which的问题，请英语好的人帮帮忙。

第一句： 这句话直接说就是 had been away FROM his hometown for twenty years
第二句： many significant advances were made in both science and philosophy IN The seventeenth century
不加介词的话很容易就看出语法错误了。
以后遇到类似的句子，可以采用这样的办法。把句子顺过来，就可以看出介词是否该放和该放哪个介词。

1、其实这是个定语从句。该句是这样的：He came back to his hometown.He had been away from his hometown for twenty years.
所以合成定语从句时，介词from不可以省略。from which 可以用where代替。

2、第二个问题的实质和第一题是一致的。分解出来之后再合成，很明显介词in不可以省略。

第一个。从句改成两个单句。
he came back to his hometown.
he had benn away from his hometown for twenty years.
which是代名词。就是替代名词的意思。
from which可以换成where。
这句提取his hometown时多出的from剩在了which前面。
第二个。强调倒装。把它正过来。
many significant advances were made in both science and philosophy
in the seventeenth century.
which仍旧是代名词。这句要强调the seventeenth century时多出的in不能忽略，所以剩在了which前面。
解释的不专业，但我一直这么理解的。楼主见笑了。