Back off 退后
Back off！Don’t touch me！
Step back 退后
Please step back from the injured woman. Give her some air.
Keep back! 退后
"Keep back!" he shouted, "Don't come any closer!”
Back and forth
During the party, I went back and forth to the kitchen to get drinks for the guests.
Back in the day
Back in the day, I used to spend my summers exploring or playing with friends. Now I'm in the office every day.