主要看从句所缺少的成分.
因为关系代词就是代替所缺成分的.
比如
1.This is the place which we visited.
这里的which在它所引导的定语从句中作宾语.
句子由主谓宾构成,把从句恢复成完整的句子就是
we visited the place.
因为从句we visited缺少宾语,所以由which充当.
2.He bought a coat that/which cost little.
这里的that/which在定语从句中作主语.
从句that cost little修饰先行词coat.
从句还原成完整句子就是A coat cost little.