English expressions and idioms with the word 'Let' 

Let someone know (about something)
To tell someone something; to inform someone of something. 

Example : Please let me know about it soon. Will you be coming to the picnic? Please let me know.

Let's Say

Used to introduce a suggestion or possible example of something.

Example :Try and finish the work by, let's say, Friday.

Let's see (spoken)

It means you are thinking about it, you want to discover more of it.

Example : “Do you have any idea how many people will be there?” “Well, let's see – I don't think I could put a number on it.”

let me see (or think)

Used when one is trying to remember something or considering one's next words.

Example : "Now let me see, where did I put it?"

Let's call it a day.

Let us end what we are doing for the day. 

Example Mary: Well, that's the end of the reports. Nothing else to do. 
Sue: Let's call it a day. 
Bob: Let's call it a day. I'm tired. 
Tom: Me too. Let's get out of here.

Let's shake on it.
To agree on something by shaking hands on it. 

Example : Bob: Do you agree? 
Mary: I agree. Let's shake on it. 
Bob: Okay. 
Bill: Good idea. Sounds fine. 
Bob (extending his hand): Okay, let's shake on it. 
Bill (shaking hands with Bob): Great!

Let it (things) slide

To allow a situation to become slowly worse. 

Example : We've really let things slide over the past few months. The accounts are in a terrible state.

let someone in on (something)

To allow to know about something, allow to share something.

Example : I'm going to let you in on a little secret.

Let go

To allow someone or something to escape or go free.
Example : The kidnappers they let the hostages go.

Let your guard down
To stop guarding oneself against trouble. 

Example : He never lets his guard down because he trusts no one.