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.

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。

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。