1. (To) sound like a broken record 

This common idiom means that someone keeps saying the same thing over and over again. Let see the following examples: 

• The boy’s mother sounds like a broken record when she tells him to clean his room. 
• He’s always complaining about the way she treats him. He sounds like a broken record! 

2. (To) Blow the whistle on someone/something

If you blow the whistle on someone, or on something secret or illegal, you tell another person, especially a person in authority, what is happening. Let see the example below.

• The employee blew the whistle on the illegal practices of the company. 
• I knew my company was polluting the water, but I was afraid I would lose my job if I blew the whistle. 

3. (To) Hit the books

This idiom means to study hard or with a concentrated effort for something. In this expression, hit means to start or to begin. For example: 

• Sorry but I can’t go to the cinema with you tonight, I have to hit the books. I have a huge exam next week.

4. To hit the sack

To hit the sack or hit the hay means to go to bed or to go to sleep. Remember to use this expression with someone you know them well. Let see the following examples:

• I am really tired after all that exercise. I am going to hit the sack.
• After the long road trip, we were all dead tired and ready to hit the hay as soon as we reached home.

5. (To be) Up in the air

If a matter is up in the air, it is uncertain, often because other matters have to be decided first. For example:

• A: Have you set a date for the wedding yet? 
• B: Not exactly, things are still up in the air and we’re not sure if our families can make it on the day we wanted. Hopefully we’ll know soon and we’ll let you know as soon as possible.

6. (To) Stab someone in the back

If someone stabs you in the back, he or she will deceive or betray someone or do harm to someone who trusted you. For example:

• I have done everything for her, but she left me because of that guy, she stabbed in my back. 
• I believed that she was my friend. I can’t believe that she stabbed me in the back.
• You need to watch out for the new kid. I heard that he will stab you in the back without a second thought.