定语从句：It is the knife that made you out of trouble.
主语从句：Whether that is possible does not have any needs to be judged by the manager.
宾语从句：I think that is a good idea.
表语从句：The problem is what that costs.
同位语从句：The question whether that is a UFO has just been discussed.
补足语从句：I do not know whether that is dirty,but I made the city where that is everywhere.（注意后半句才含有宾语补足语从句）
状语从句：If that is nice,I will introduce that to my friend.
但只有定语从句中的that才可能既做主语,又是引导词.