If you let someone down, you disappoint them, by not doing something that you have said you will do or that they expected you to do.

If something lets you down, it is the reason you are not as successful as you could have been.

If an object lets in something such as air, light, or water, it allows air, light, or water to get into it, for example, because the object has a hole in it.

If someone in authority lets you off a task or duty, they give you permission not to do it.

If you let someone off, you give them a lighter punishment than they expect or no punishment at all.

If you let off an explosive or a gun, you explode or fire it.

Let out means the same as letverb:transitive10.

If something or someone lets water, air, or breath out, they allow it to flow out or escape.

If you let out a particular sound, you make that sound.

If you let out a dress or pair of trousers, you make it larger by undoing the seams and sewing closer to the edge of the material.

If an unpleasant, continuous process lets up, it stops or becomes less intense.

Let alone is used after a statement, usually a negative one, to indicate that the statement is even more true of the person, thing, or situation that you are going to mention next.

If you let go of someone or something, you stop holding them.

If you are let down by someone or something, you are disappointed because they did not do something that they said they would do or that you expected them to do.

If you let someone or something go, you allow them to leave or escape.

When someone leaves a job, either because they are told to or because they want to, the employer sometimes says that they are letting that person go.

If you say that you did not know what you were letting yourself in for when you decided to do something, you mean you did not realize how difficult, unpleasant, or expensive it was going to be.

If you let someone know something, you tell them about it or make sure that they know about it.

If you let something happen, you allow it to happen without doing anything to stop or prevent it.

If you let someone do something, you give them your permission to do it.

If you let someone into, out of, or through a place, you allow them to enter, leave, or go through it, for example, by opening a door or making room for them.

You use let me when you are introducing something you want to say.

You use let me when you are offering politely to do something.

You say let's or, in more formal English, let us, to direct the attention of the people you are talking to towards the subject that you want to consider next.

You say let's or, in formal English, let us, when you are making a suggestion that involves both you and the person you are talking to, or when you are agreeing to a suggestion of this kind.

Someone in authority, such as a teacher, can use let's or, in more formal English, let us, in order to give a polite instruction to another person or group of people.

You can use let when you are saying what you think someone should do, usually when they are behaving in a way that you think is unreasonable or wrong.

If you let your house or land to someone, you allow them to use it in exchange for money that they pay you regularly.