back and forth = to go between two places, again and again.

He bikes back and forth between his job and his apartment every day.

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."