一不做，二不休
in for a penny, in for a pound
He was found stealing money from his landlady by her daughter. "In for a penny, in for a pound," he muttered to himself and then killed the little girl.
 
一文不名；一贫如洗
not have (or without) a penny to one's name; not have two halfpennies to rub together; (as) poor as a church mouse; not have a bean; not have (or without) a red cent
When I got married I didn't have a penny to my name.
That family never seem to have two halfpennies to rub together.
I am sorry, I can't lend you anything -- I haven't got a red cent.
 
一石二鸟；一箭双雕；一举两得
kill two birds with one stone
Since Benjamin, one of my classmates in university, lives near my mother, I'll call in on him as well and kill two birds with one stone.

一帆风顺
plain sailing
The security situation in the Taiwan Strait is far from being plain sailing.

一见如故
get on like a house on fire (英) / get along like a house on fire (美)
If two people get on/along like a house on fire, they like each other very much and become friends very quickly.
I was worried that they wouldn't like each other, but in fact they're getting on/along like a house on fire.

 
一文不值
not worth a damn
His promise isn't worth a damn.

一针见血；一语中的
hit the nail on the head
He hit he nail on the head when he mentioned casual sex as being the cause of her coming down with AIDS.

一言以蔽之
in a nutshell
There's a lot I could say about my boss's speech but to put it in a nutshell, it was terrible.

一命呜呼
turn up one's toes
I hope to take a trip to the United States before I turn up my toes.

一丝不苟
dot one's/the i's and cross one's/the t's
You'd better make sure you dot your i's and cross your t's if you want to work at that company. They're very keen on details.

一丝不挂
in a state of nature; in the altogether; in the buff; not have a stitch on (or without a stitch on)
I had just got out of the bath and still was in the buff (or in the altogether/in a state of nature) when the maid walked in without knocking.
I had just got out of the bath when he knocked at the door and I didn't have a stitch on.

一模一样
as like as two peas (in a pod)
If two brothers or two sisters are very much alike, you can say they are as like as two peas in a pod.

一鸣惊人；一举成名；不同凡响
set the Thames on fire (英) / set the world on fire (美)
Mary may set the Thames on fire with her piano playing.
His great invention set the world on fire.

一筹莫展
up the creek (without a paddle)
He was really up the creek when his wife left him.

一丘之貉；物以类聚
birds of a feather (flock together); be tarred with the same brush
We all thought that he and his girlfriend were birds of a feather.
I dislike politicians of all parties because they're all tarred with the same brush.

一箭之遥；近在咫尺
a stone's throw
My house is only a stone's throw from the school.

一意孤行；我行我素；独断独行
go/take one's own way
I warned him not to marry that girl, but he was determined to go his own way.

 
一知半解
a little knowledge
A little knowledge is a dangerous thing. (这一句是颇有名的谚语，意为「一知半解最危险；半瓶子醋害死人」)

一见钟情
fall in love (with someone) at first sight; love at first sight  (这里 "love" 为名词)
They fell in love with each other at first sight.
They fell in love at once; it was love at first sight.

一清二楚
(as) clear as crystal; (as) clear as a bell
I don't know why you can't understand the instructions; they're as clear as crystal to me.
The whole thing was as clear as a bell to me. I knew what to do.

一窍不通
be all Greek to someone
He tried to explain how the computer worked, but it was all Greek to me.

一白遮三丑
A white complexion is powerful enough to hide seven faults.
A white complexion is powerful enough to hide seven faults.

一朝被蛇咬，十年怕草绳；上一次当，学一次乖
once bitten, twice shy
He has cheated and deceived me on one occasion, so I don't trust him. Once bitten, twice shy.