它们之间可以有条件的进行相互转换。
有些是可以当作省略了来看
但有些要变换时态的，
所以并不是单纯的省略。
举个例子吧
Seeing those pictures．he couldn't help thinking of the unforgettable days in
New York．(时间状语)
→When he saw those pictures．he eouldn"t help thinking of the unforgettable days in New Y0rk．
They Started off early in order to(so as to)arrive
in time．
→They started of early in order that(so that)they could arrive in time．
这个语法比较繁琐的，抽空多看看语法书，有专门讲的。