a word where和a word which的不同之处

请问一下老师, 这句子中两处都用where而不是which这是哪些原因,"word"它在从句中不作成份吗?  第二个where前还加了逗号,这是必需的吗,还是说这依据习惯比较随便的?

I can't think of a word where the pronunciation would bedifferent,except a compound word like FactTime,where there is some stress oneach syllable..

网友zhangh 应该是本站的老会员了，如何会提这么幼稚的问题呢？

下面叫你不同这两个句子，为什么一个用 where,一个用 which：
I can't think of a word where the pronunciation would be different.
I can't think of a word which would be different from this.

除此之外，定语从句前面有逗号，这有哪些奇怪的？ 缘由：要么由于先行词前后都有逗号，需要停顿，要么就是非限制性的。