最簡(jiǎn)單的方法：看從句缺什么成分.
比如：
1.He is the policeman who caught the thief the other day.
去掉who后,從句缺主語：caught the thief the other day.所以who充當(dāng)主語.
2.He is the policeman that I met yesterday.
去掉that后,從句缺賓語：I met yesterday.所以that充當(dāng)賓語.
3.I like the book whose cover is dark green.
去掉whose后,從句缺定語：(Its) cover is dark green.故whose充當(dāng)定語.
4.The temple where he used to live has been destroyed.
去掉where后,從句缺狀語：He used to live (in the temple).故where充當(dāng)狀語.
如果僅僅是想?yún)^(qū)分主語或賓語,還有一種簡(jiǎn)便的方法,就是看關(guān)系詞后是什么詞性的詞,后面是名詞（主語）的話先行詞就是賓語,是動(dòng)詞（謂語）的話先行詞就是主語.
如：
1.Father is talking to the man that has stayed here for a long time.
句中that后是謂語動(dòng)詞,所以that就充當(dāng)主語,可以用who來代替.
2.Father is talking to the man that we are afraid of.
句中that后是名詞（主語）,所以that就充當(dāng)賓語,可以用whom來代替.