|back and forth = to go between two places, again and again.
He bikes back and forth between his job and his apartment every day.
|back up = to support, to help someone
Robert plans to back up his wife when she delivers her baby. (used as an infinitive)
If he is unable to drive her to the hospital, they have a back up plan. His brother can drive her there. (used as an adjective)
|be at it = to do something; to practice or learn how to do something.
A: They love to play with those blocks.
B: Yeah, they've been at it for the last hour.
(Note: This idiom is often used in the present perfect tense)
|bend over backwards = to work hard at something; to try to help someone.
The government bent over backwards to get the I-35W bridge rebuilt.
|better off = in a situation that is good--better than before.
Jen thinks she's much better off with her new boyfriend. Her old boyfriend wasn't very nice to her.
|break down = a machine stops working, usually used for cars
Tim's car breaks down all the time. He has to get a new one.
This is the third time it has broken down this week. (present perfect)
|bring up = to say something; to introduce a topic for conversation.
If you have chest pains that don't go away, you should probably bring that up with your doctor next time you see him.
|break up = 1. to make something small; 2. to end a relationship.
She got a phone call from her boyfriend. He told her he wanted to break up with her. (definition #2)
|bump into = meet someone you know by chance or by accident.
On the way home from the store, Alice bumped into an old friend she hadn't seen in years.
|by the way = This idiom is used when you want to introduce something new in a conversation. It's usually off topic.
"Juggling is easy for me. I've been juggling for years. By the way, I also know how to do backwards somersaults."