Know can be used as a noun in a couple of expressions.

If you are in the know you have been told confidential information.

For those of you not already in the know, Jane and I are getting married.

If you have know-how. you have expertise.

Does he have the technical know-how to work on this project?

But, more usually, know is a verb and can be followed by objects, that-clauses or wh-clauses.

I know John really well. We used to work together.

I know that you are unhappy. Can we talk about it?

I know why we are doing so badly. It’s our poor website.

You know can be used to emphasise a point.

You are not helping yourself, you know, by always arguing with your boss.

But, more frequently, you know is used as a way of pausing to give the speaker time to think. Many people find this very irritating and you should not try to do this yourself 😉

I went to see him, you know, and he was, you know, very angry. So I said I was, you know, sorry to try to, you know, calm him. 

Before you know it means very quickly.

Two months seems like a long time but it will be over before you know it.

Don’t I know it is an informal way of agreeing with what has just been said.

Don’t I know it. You are quite right. 

For all I know means that you have no knowledge of something.

For all I know, they might not even be interested in the project.

I don’t know can be used to express doubt or disagreement.

I don’t know that I agree. I can see the other side of the argument.

I don’t know about John. I am not sure he is capable.

If you must know is used when you are giving information that is personally embarrassing.

If you must know, I am divorced. 

I know can be used to express agreement.

I know. You are quite right.

I know can be used to introduce a suggestion.

I know. Let’s go out for a meal. 

I wouldn’t know is used to say that you have no experience of something.

I wouldn’t know anything about it. I have never done that before.

If you know better/best, you know about something more than anybody else. It is often used in a sarcastic way.

He always thinks he knows best. He is very irritating.

If  you are sensible enough not to do something, you know better.

I know better than to believe a word he says.

If you know that something that somebody says is inaccurate, you know better.

She told me that John was certain to get the job but I knew better.

If you don’t know what hits you, you are taken completely by surprise.

When we brought out the new version, our competitors didn’t know what had hit them.

If you know what it is, you have experience of something.

I know what it is to live on a very low income.

If somebody knows what they are talking about, this person is an expert and deserves to be listened to.

He certainly knows what he is talking about. We should do what he says.

If you don’t know the first thing about something, you are completely ignorant about it.

I don’t know the first thing about networks. Could you explain this to me in simple terms?

You never know is used to say that we can never be totally sure about the future.

You never know what will happen.