过去将来时的基本特征，也就是我么年今天所谈到的“立足过去，着眼未来”。它表示从过去某一时间来看将要发生的某个动作或存在的某种状态。例如：Mary told me that she would go to Shanghai by plane.那么过去将来时态的构成是什么呢？它都有哪些用法呢？
A ) would + 动词原形
如He asked me if I would stay here. 他问我是否要待在这儿。
B ) was / were going to + 动词原形
如No one knew when he was going to finish his homework.
C ) was/ were ( about ) to + 动词原形
如：He said that they were to leave at six.
She said that the meeting was about to begin.