1、 在“介词＋关系代词”结构中，关系代词指物必须用which，指人必须用whom，而不能用that。
This is the film about which we are talking . 这就是我们正在谈论的电影。
This is the film star about whom we are talking . 这就是我们正在谈论的电影明星。
如果介词不放在修饰人（或事物）的限定性定语从句的句首，whom或which就可换为that。
即：This is the film which/that we are talking about.
This is the film star whom/that we are talking about.
2.在非限制性定语从句中只能使用关系词which，不能使用关系词that。The old tree, which is four hundred years , is very famous here.
那棵古树已经有几百年的岁月了，在这里很有名。
3.先行词为“those＋表事物的复数名词”时， 关系代词通常只用which而不用that。如：You should learn those skills which may be useful in the future.
你们应该掌握那些将来用得着的技能。
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