fail和fail in用法什么时候加in什么时候不加in,

fail the exam 是考试不及格,fail in the exam 是在考试中失败,两者不一样
fail
[feIl]
vi,vt
失败；不及格
The teachers failed me on the written composition.
教师们评我的笔头作文不及格.
He failed his English examination.
他的英语考试不及格.
不足；无法做到；（作物）歉收
Last year the crops failed.
去年农作物歉收.
"They fail to recognize that our population is increasing faster than the supply of food,available resources and job opportunities."
他们没有认识到我们人口的增长超过了粮食的供给|资源的开发和就业的机会.
使失望；辜负
His courage failed him in the end.
"到最后,他泄气了."
We will never fail to live up to what our parents expect of us.
我们决不辜负我们的父母对我们的期望.
His friends failed him when he most needed them.
当他最需要朋友的时候,他们却令他失望.
衰退；衰弱
The wind failed and the sea returned to calmness.
"风停了,海面上恢复了一片寂静."
（事业）倒闭,破产