Question

It is the room ______ we’ll have the party. 为什么填where而不是which

Answer 1

因为，该题目中关系词在从句中充当的是状语成分。
我们选择合适的关系词的关键是，关系词在定语从句中所充当的成分。
如果定语从句中谓语动词为不及物动词，或者是带有宾语的及物动词，那么可以判定为缺少状语。那么就要选择关系副词。
例如：This is the factory where he works.(work为不及物动词)，这里选择关系副词where。
比较：This is the factory which /that he visited.(定语从句中谓语动词visit是及物动词，而且缺少宾语，关系词选择which或者that)。

该题目容易看成是强调句式，强调句一般是强调主语、宾语和状语。该题目中的the room不符合这种情况。如果讲the room 改成in the room,那么改成：“It is in the room that we'll have the party.”

Answer 2

we will have the party in the room.
it is the room where/in which we will have the party

Answer 3

where用来引导定语从句，在从句中作地点状语。先行词是the room，定语从句的引导词选where还是which取决于定语从句中缺什么成分：缺宾语需考虑which，不缺宾语只能考虑where。