什么情况下关系代词只能用that，而不用which

1） 当先行词是：all , any , anything , everything , nothing或被它们修饰时。例如：
Is there anything that I can do for you ? 有什么可以为你做的吗？ All that can be done must be done . 凡是能做的都必须做。
2）当先行词被形容词的最高级或序数词修饰时。例如： That is one of the most interesting books that are sold in the bookshop . 那是这家书店出售的最有趣的书之一。 The first thing that we should do is to get some food . 我们该做的第一件事是弄点吃的。
3）当先行词有：the only , the very , the same , the last 等修饰时。例如： My necklace is not the only thing that's missing .
我的项链不仅是丢掉的东西。
4）当主句以who或which开头时，定语从句中引导词用that ,不用which 或 who 。以避免重复。例如：
Who is the girl that is wearing a red coat ? 正穿一件红色外套的女孩子是谁？
5）当先行词既有人又有物时，用that 。