You use only to indicate the one thing that is true, appropriate, or necessary in a particular situation, in contrast to all the other things that are not true, appropriate, or necessary.

You use only to introduce the thing which must happen before the thing mentioned in the main part of the sentence can happen.

If you talk about the only person or thing involved in a particular situation, you mean there are no others involved in it.

An only child is a child who has no brothers or sisters.

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

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

Only is used after 'can' or 'could' to emphasize that it is impossible to do anything except the rather inadequate or limited action that is mentioned.

You can use only in the expressions I only wish or I only hope in order to emphasize what you are hoping or wishing.

Only can be used to add a comment which slightly changes or limits what you have just said.

Only can be used after a clause with 'would' to indicate why something is not done.

You can use only before an infinitive to introduce an event which happens immediately after one you have just mentioned, and which is rather surprising or unfortunate.

You can use only to emphasize how appropriate a certain course of action or type of behaviour is.

You can use only 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.

If you say you only have to or have only to do one thing in order to achieve or prove a second thing, you are emphasizing how easily the second thing can be achieved or proved.

You can say that something has only just happened when you want to emphasize that it happened a very short time ago.

You use only just to emphasize that something is true, but by such a small degree that it is almost not true at all.

You can use only too to emphasize that something is true or exists to a much greater extent than you would expect or like.

You can say that you are only too happy to do something to emphasize how willing you are to do it.