325.What has happened is that every time a material seems to be running short, the price has risen and, in response, people have looked for new sources of supply, tried to find ways to use less of the material, or looked for a new substitute.
本句的主干是What has happened is that...。What has happened为主语从句，that引导的是表语从句that every time...new substitute；every time为该表语从句的时间状语，而a material seems to be running short则为省略关系副词when的定语从句修饰time the price has risen and，in response，people have...a new substitute为该表语从句的主干。
In response，the restaurant began offering more seating.
I have run short of money.
She is seeking a substitute for the man who broke her heart.
That he will come here on Tuesday is certain.他周二会来这里是确定的事实。可以改写成：It is certain that he will come here on Tuesday.
What he told me was true.他告诉我的是真的。不胄瓯兑：It was true what he told me.