back down (no object): not follow a threat.
"Tom was going to call the police when I told him I'd wrecked his car, but he backed down when I said I'd pay for the damages."
"Tom was ready to call the police when I told him I'd wrecked his car, but he backed offwhen I said I'd pay for the damages."back up (1. no object): move backward; move in reverse.
"You missed the lines in the parking space. You'll have to back up and try again."
"The people waiting in line are too close to the door. We won't be able to open it unless they back up."
"You're too close! Back your car up so I can open the garage door."
"If you don't believe me, talk to Dave. He'll back me up."
"When my computer crashed, I lost many of my files. It's a good thing I backed themup."
"At first Lily said she would be at the party. Later she begged off."
"We needs lots of balloons for the party. Will you blow them up?"
A: "That old building really came down quickly!"
B: "That's because the construction company used dynamite to blow it up."
"When I told Jerry that I'd had an accident with his car, he blew up."
"If you're going to travel to Peru, you'd better bone up on your Spanish."
"We spent a lot of money at the supermarket. When we broke the total cost down, we spent more on cleaning supplies than food."
"Sharon will be late for work today. Her car broke down on the freeway."
"Jane's apartment was burglarized last night. Someone broke in while Jane was at the movies." / "Somebody broke into Jane's apartment while she was at the movies.break in (2. separable): wear something new until it's / they're comfortable.
"These are nice shoes, but they're too stiff. I hope it doesn't take too long to break themin."break in (3. separable): train; get someone / something accustomed to a new routine.
"I hope I can learn my new job quickly. The manager hasn't scheduled much time forbreaking me in."