1.He is the policeman who caught the thief the other day.去掉who后,从句缺主语：caught the thief the other day.所以who充当主语.
2.He is the policeman that I met yesterday.去掉that后,从句缺宾语：I met yesterday.所以that充当宾语.
3.I like the book whose cover is dark green.去掉whose后,从句缺定语：(Its) cover is dark green.故whose充当定语.
4.The temple where he used to live has been destroyed.去掉where后,从句缺状语：He used to live (in the temple).故where充当状语.