onto

(MOVEMENT) used to show movement into or on a particular place
(CHANGING SUBJECT) used about changing to, or starting to talk about, a different subject
(HOLDING) to keep holding something
(KNOWING) knowing about someone or something that can be useful to you
(ASKING) If you are onto someone, you talk to that person, especially to ask them to do something, or to complain to them.
(ADDING) used about someone or something that is added to or joins a particular thing

If something moves onto or is put onto an object or surface, it is then on that object or surface.
You can sometimes use onto to mention the place or area that someone moves into.
You can use onto to introduce the place toward which a light or someone's look is directed.
If a door or room opens onto a place, that place is directly in front of it.
When you change the position of your body, you use onto to introduce the part your body which is now supporting you.
When you get onto a bus, train, or plane, you enter it in order to travel somewhere.
Onto is used after verbs such as "hold," "hang," and "cling" to indicate what someone is holding firmly or where something is being held firmly. (hold)(hang)(cling)
If people who are talking get onto a different subject, they begin talking about it.
You can sometimes use onto to indicate that something or someone becomes included as a part of a list or system.
If someone is onto something, they are about to discover something important.
If someone is onto you, they have discovered that you are doing something illegal or wrong.

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