一朝被蛇咬，十年怕草绳
Once bitten, twice shy.
一言既出，驷马难追
A word spoken is past recalling.
一见钟情
to fall in love at first sight
一箭双鵰/一举两得
Kill two birds with one stone.
一寸光阴一寸金
Time is money.
一失足成千古恨
The error committed on impulse may turn out to be the sorrow of a whole life.
一将功名万古枯
What millions died that Caesar might be great.
一年之计在于春，一日之计在于晨
Plan your year in spring and your day at dawn.
一人难称百人意
You cannot please everyone.
一气呵成
Never make two bites of a cherry.
on impulse：一时冲动，一时心血来潮
turn out：结果是， 证明是