一般式：情态动词（can,could,may,might,must,should,need等）+be+过去分词
完成式：情态动词（can,could,may,might,must,should,need等）+have+been+过去分词
例句：
Lost health may be replaced by temperance or medicine.
失去的健康可以靠节制或药物而复得。
This can be done by hand.
这可以手工做。
This mustn't be neglected.
这一点不可忽视。
It needn't be mentioned in your talk.
这一点你在谈话中不必提及。
The environment should be improved.
环境应当改善。
The project might have been completed earlier.
这项工程本可以早点完工的。
Such a situation could have been changed.
这种情况本可以改变的。
They shouldn't have been told about the plan.
这个计划是不应当告诉他们的。
