a. 用于由if,if only,as if 引导的分句中 If only you had told me before.
b.用于I wish ,I'd rather等结构后面的that-分句中 I wish I had said that I couldn't come.
c. 用于expect,think,intend等动词表示过去未曾实现的希望、打算或意图。I had hoped that we would be able to leave tomorrow,but it's beginning to look difficult.