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."

back off (no object): not follow a threat.

"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."

back up (2. separable): drive a vehicle backwards (in reverse).

"You're too close! Back your car up so I can open the garage door."

back up (3. separable): confirm a story, facts, or information.

"If you don't believe me, talk to Dave. He'll back me up."

back up (4. separable): make a "protection" copy to use if there are problems with the original.

"When my computer crashed, I lost many of my files. It's a good thing I backed themup."

beg off (no object): decline an invitation; ask to be excused from doing something.

"At first Lily said she would be at the party. Later she begged off."

blow up (1. separable): inflate.

"We needs lots of balloons for the party. Will you blow them up?"

blow up (2. separable): explode; destroy by exploding.

A: "That old building really came down quickly!"
B: "That's because the construction company used dynamite to blow it up."

blow up (3. no object): suddenly become very angry.

"When I told Jerry that I'd had an accident with his car, he blew up."

bone up on (inseparable): review / study thoroughly for a short time.

"If you're going to travel to Peru, you'd better bone up on your Spanish."

break down (1. separable): separate something into component parts.

"We spent a lot of money at the supermarket. When we broke the total cost down, we spent more on cleaning supplies than food."

break down (2. no object): stop working / functioning.

"Sharon will be late for work today. Her car broke down on the freeway."

break in (1. often no object; with an object, break into--inseparable): enter by using force (and breaking a lock, window, etc.)

"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."