What's the difference between "onto" and "on to" and where should they be used, etc?
"On to" implies that you are removing an object from one place to another, either physically, or conceptually. "Onto" refers to the positioning or placement of something, so that it physically touches the object. To help make this distinction, place a comma in the sentence.
I will go on, to the rock.
This tells the reader that the speaker plans to travel, until he arrives within close proximity of the rock. The speaker said nothing about making physical contact with the rock. Once the speaker arrives, they may declare,
I am going onto the rock.
At this time, the speaker indicates they are about to physically tough the rock, and stand upon it.
The "into" comment is also a matter of action versus position. Also, consider the prepositional context. This statement makes no sense, out of context:
I will go in, to the rock.
This means the person is going the the inside of something (cave, tunnel, etc.), to the location of the rock. More information about the location of the rock is needed.
Logically, one can not go "into" a rock, which implies the speaker is going to travel to the inside of the rock, for two reasons: first, there is no "inside" of a rock, as it is a solid; and second, the rock is kn fact solid. The concept of going into a rock is nonsense. Well, unless you're abusing hallucinogens.
However, liquids are different. You can logically state,
I am diving into the ocean. I am walking into a cloud. I am staring into the abyss.
In these cases, substituting "in to" just doesn't work, because these things are massive and are unable to be contained in a reasonably finite space. Therefore, saying you are "walking in, to the ocean" doesn't make much sense. If anything, you're far more likely to be "walking out, to the ocean." If you were already in the ocean, and the subject were walking toward you, from land, then this could well make sense, but is rarely the case.
The New Oxford American Dictionary has:
USAGE: The preposition onto written as one word (instead of on to) is recorded from the early 18th century and has been widely used ever since. In U.S. English, it is the regular form, although it is not wholly accepted in British English. Nevertheless, it is important to maintain a distinction between the preposition onto or on to and the use of the adverb on followed by the preposition to: she climbed onto (sometimes: on to) the roof, but: let's go on to (never onto) the next chapter.