You use just to say that something happened a very short time ago, or is starting to happen at the present time. For example, if you say that someone has just arrived, you mean that they arrived a very short time ago.

If you say that you are just doing something, you mean that you are doing it now and will finish it very soon. If you say that you are just about to do something, or just going to do it, you mean that you will do it very soon.

You can use just to emphasize that something is happening at exactly the moment of speaking or at exactly the moment that you are talking about.

You use just to indicate that something is no more important, interesting, or difficult, for example, than you say it is, especially when you want to correct a wrong idea that someone may get or has already got.

You use just to emphasize that you are talking about a small part, not the whole of an amount.

You use just to emphasize how small an amount is or how short a length of time is.

You can use just in front of a verb to indicate that the result of something is unfortunate or undesirable and is likely to make the situation worse rather than better.

You use just to indicate that what you are saying is the case, but only by a very small degree or amount.

You use just with 'might,' 'may,' and 'could', when you mean that there is a small chance of something happening, even though it is not very likely.

You use just to emphasize the following word or phrase, in order to express feelings such as annoyance, admiration, or certainty.

You use just with instructions, polite requests, or statements of intention, to make your request or statement seem less difficult.

You use just in expressions such as just a minute and just a moment to ask someone to wait for a short time.

You can use just in expressions such as just a minute and just a moment to interrupt someone, for example in order to disagree with them, explain something, or calm them down.

You can use just with negative question tags, for example 'isn't he just?' and 'don't they just!', to say that you agree completely with what has been said.

If you say that you can just see or hear something, you mean that it is easy for you to imagine seeing or hearing it.

You use just to mean exactly, when you are specifying something precisely or asking for precise information.

You use just to emphasize that a particular thing is exactly what is needed or fits a particular description exactly.

You use just in expressions such as just like ,just as...as, and just the same when you are emphasizing the similarity between two things or two people.

You use just about to indicate that what you are talking about is so close to being the case that it can be regarded as being the case.

You use just about to indicate that what you are talking about is in fact the case, but only by a very small degree or amount.

Just on is used in mentioning an almost exact number or amount.

Just so is used to agree with or confirm a statement that has been made.

If things are just so, they are done or arranged exactly as they should be or exactly as someone wants them.

You use the expression it's just that when you are making a complaint, suggestion, or excuse, so that the person you are talking to will not get annoyed with you.