1. hang by a hair

释义：To be perilously close to failing, dying, or resulting in a bad outcome. （濒临失败、濒于死亡或导致坏结果，岌岌可危的情况）

用法：也可以说为：hang on by a hair。

例句：e.g. As Jack's car ran into a truck his life hung by a hair.

杰克的车撞上卡车时，他的生命危在旦夕。

e.g. After the loss, their chances of getting into the championships are hanging on by a hair.

输掉这场比赛后，他们参加锦标赛的机会微乎其微。

e.g. The future of this company hangs by a hair. Unless we get two or three big orders by the end of the month, we’re finished.

我们的公司濒临破产。除非我们在月底得到两个或三个大订单，否则我们就完蛋了。

2. hang by a thread

释义：To be perilously close to failing, dying, or resulting in a bad outcome. （濒临失败、濒于死亡或导致坏结果，岌岌可危的情况）

例句：e.g. After the operation, his life hung by a thread for several hours.

手术后，他的生命危在旦夕，或许只能活几个小时。

e.g. Her life hung by a hair as medics rushed her to the hospital.

她的生命危在旦夕，医护人员火速将她送往医院。

e.g. John isn't failing in English, but his passing grade is just hanging by a hair.

约翰的英语考试及格了，但是差点没有及格。

3. by a hair

释义：By an extremely short or slim margin (of distance, time, or another measure). （差一点儿）

用法：也可以说 ：by a hair breadth。

例句：e.g. The bullet missed him by a hair.

枪弹擦身而过，差点打中他了。

e.g. The ball missed his head by a hair.

球差一点击中他的头。

e.g. It was as close as an election could be. A million people voted and the candidate I voted for won by only 350 votes. She only won by a hair but that still makes her our new governor.

这次选举竞争空前激烈，有一百多万人投票。他选的侯选人仅仅领先三百五十票而赢得胜利，当上了州长。