You say 'I know' to show that you agree with what has just been said.

You can use expressions such as you know what I mean and if you know what I mean to suggest that the person listening to you understands what you are trying to say, and so you do not have to explain any more.

You say 'You never know' or 'One never knows' to indicate that it is not definite or certain what will happen in the future, and to suggest that there is some hope that things will turn out well.

You say 'Not that I know of' when someone has asked you whether or not something is true and you think the answer is 'no' but you cannot be sure because you do not know all the facts.

You use you know to emphasize or to draw attention to what you are saying.

If you talk about a thing or system as we know it, you are referring to the form in which it exists now and which is familiar to most people.

If you get to know someone, you find out what they are like by spending time with them.

People use expressions such as goodness knows, Heaven knows, and God knows when they do not know something and want to suggest that nobody could possibly know it.

You can use I don't know to indicate that you do not completely agree with something or do not really think that it is true.

You can say 'I don't know about you' to indicate that you are going to give your own opinion about something and you want to find out if someone else feels the same.

You use I don't know in expressions which indicate criticism of someone's behaviour. For example, if you say that you do not know how someone can do something, you mean that you cannot understand or accept them doing it.

If you are in the know about something, especially something that is not known about or understood by many people, you have information about it.

You can say 'You don't know' in order to emphasize how strongly you feel about the remark you are going to make.

If you know a fact, a piece of information, or an answer, you have it correctly in your mind.

If you know someone, you are familiar with them because you have met them and talked to them before.

If you say that you know of something, you mean that you have heard about it but you do not necessarily have a lot of information about it.

If you know about a subject, you have studied it or taken an interest in it, and understand part or all of it.

If you know a language, you have learned it and can understand it.

If you know something such as a place, a work of art, or an idea, you have visited it, seen it, read it, or heard about it, and so you are familiar with it.

If you know how to do something, you have the necessary skills and knowledge to do it.

You can say that someone knows that something is happening when they become aware of it.

If you know something or someone, you recognize them when you see them or hear them.

If someone or something is known as a particular name, they are called by that name.

If you know someone or something as a person or thing that has particular qualities, you consider that they have those qualities.