Hard and fast ( বাঁধা ধরা ) There is no such hard and fast rule in this matter.
A blessing in disguise ( বিপরীতে হিত ; প্রথমে খারাপ হিসেবে মনে হলেও পরবর্তীতে ভালো হয় ) Sam's motorcycle accident was a blessing in disguise because he got enough insurance money.
A rainy day ( দুর্দিন ) Everybody should save something for rainy day.
To the backbone ( হাড়ে হাড়ে ) The boy is wicked to the backbone.
By chance ( দৈবাৎ ) I met him on the way by chance.
Rank and file ( সাধারণ লোক ) We should pay attention to the rank and file of the country.