谨言慎行；循规蹈矩；小心行事
mind one's P's and Q's
Their grandmother often told the children to mind their p's and q's.
　
覆水难收
don't cry over spilt milk; it is no use crying over spilt milk
The papers you wanted went out in last week's trash, so don't cry over spilt milk.
　
归根结柢
when/after all is said and done
When all's said and done, the doctors did what they could for John, but he was too ill to survive.
　
转石 (或滚石) 不生苔； 转业不聚财
a rolling stone gathers no moss
Kate's lived in ten cities in as many years, so I don't think the saying "a rolling stone gathers no moss" means anything to her.
　
镇定自若；泰然自若；不动声色
keep one's countenance
Under the circumstances it is hard for him to keep his countenance.