原则：定语从句关系词的选择是根据先行词在从句中所作的成分来决定的.
where用在关系词在从句中做状语的情况,而that/which则在从句中起名词性作用.
举例：I want to go to Beijing where I can visit some places of interest.where在从句中做地点状语,指代in Beijing.
I want to go to Beijing in which I can visit some places of interest.which在从句中作in的宾语,构成介宾短语,整个介宾短语作状语.
I want to go to Beijing which/that is the capital of China.which/that在从句中作主语,指代Beijing.
不懂追问.