Well, it depends, doesn’t it?
Even if you’ve never thought about it before, it’s perhaps not too surprising that the word into is a combination of the words in and to. If you think about any sentence in which you might use the word, it clearly combines the meaning of both:
He walked into the room.
To is there because there’s movement, and to usually comes after verbs of movement. In is there because he ends up in the room. Easy. But, does this mean we can always replace in to with into?
What do you think of this sentence, that I saw today?
Carles Puigdemont has handed himself into the police.
It seemed off to me, and I quickly realised it was because of the into. I knew in this case it had to be in to, and then started to think about why.
Let’s take a quick look at the grammar. Into is a preposition, a simple type of word that connects other words in various ways. In and to can both be prepositions individually, but can also be different word types. To can also be part of the infinite form of a verb, or an adverb, and in can be an adverb too, often as part of a phrasal verb.
If we’re using both in and to as prepositions, then we can usually combine them. This counts even if we’re not talking about actually moving inside somewhere, e.g.
They played right into my hands.
If to is part of a verb though, we can’t combine them:
I looked in to see what was happening.
Here, to see is a complete grammatical unit. It’s separate from the verb-and-preposition combination looked in. It explains why we looked in. Contrast this with I looked into the room. Here into is a preposition telling us where I looked.
Sometimes, in is an adverb that forms part of a phrasal verb. For example:
He logged in to his account.
Here, logged in is a single unit, and to his account is giving us more detail. This is also the case in the headline mentioned earlier. In there is part of the phrasal verb to hand in. Therefore using into is strictly wrong in this case. It also feels odd because it makes us imagine Puidgemont actually being inside the police.
In other similar cases though, combining in and to is strictly wrong, but the result is OK. For example:
They broke into our home!
We checked into the hotel.
These seem fine, because in both cases there’s actual movement into a building involved, even if strictly in and to should be separated as in is part of a phrasal verb (to break in and to check in).
Hand into doesn’t work though, because it’s not about such movement, even metaphorically, because you hand yourself into people, not a place.
Still, it could’ve been worse: the headline could easily have said that he’d turned himself into the police. Which reminds me: have I ever told you the one about the magic tractor?