Into, or “in to”? Onto, or “on to”?
Into and onto are prepositions, words that describe relative position. They are part of prepositional phrases, such as “She settled herself into her seat” or “He climbed onto the roof.” These words are forward looking, in that, as their grammatical name implies, they are positioned before the object.
“In to” and “on to,” on the other hand, are combinations of an adverb (in or on) and the preposition to. Unlike the single-word forms, they look both backward (in and on refer to a preceding verb) and forward (to pertains to the following object).
Of the distinctions between each pair, that distinguishing into from “in to” is more straightforward. If you wish to write that you went somewhere to let a representative of a company know you are disappointed with a product or service, you can express that idea using either form. But if you write, “I walked into the office to lodge a complaint,” the sentence focuses on the prepositional phrase “into the office.” If you write, “I walked in to lodge a complaint,” the emphasis is the phrase describing the action: “I walked in.”
Onto and “on to” can be more confusing, but think of the problem this way: “She drove onto the highway” means, “She drove so that she was on the highway.” Conversely, “She drove on to the highway” means, “She headed for the highway.” The two-word form is also appropriate for figurative meanings, where no physical movement or placement exists — for example, “I think you’re really on to something.”
Fortunately, there are a couple of ways to test whether onto or “on to” is correct — temporarily insert the word up after the verb, or, just for the test, replace the word or the phrase with the word on:
The “Up” Test
When you wish to write that you used a ladder, could you write, “I climbed up onto the ladder”? Yes, you could, so onto is correct. When you want to express that you clutched something, could you write, “I held up on my hat”? No, the sentence does not make sense, so the two-word form (“I held on to my hat”) is the right one in this case.
The “On” Test
When you wish to write that you scaled a boulder, could you write, “I climbed on a boulder”? Yes, you could, so onto is correct. When you want to express that you bequeathed something, would you write, “I passed it on him”? No, that doesn’t make sense, so the two-word form (“I passed it on to him”) is correct here.