Into vs In To: which is correct?

What the one-word version does

Into, written as one word, is a preposition with two main jobs. The first is movement toward the inside of something or toward a point of contact: she walked into the room, pour the milk into the jug, he bumped into the door. The second is transformation or a change of state: the rain turned into snow, translate this into French, the project grew into something bigger. In both senses, into expresses a relationship between two things, motion inward or a change from one form to another, in a single word.

Into also lives inside many common phrasal expressions: run into someone (meet by chance), look into a matter (investigate), get into trouble, be into a hobby (interested in it), talk someone into something (persuade). In all of these, into is one word because it is functioning as a unit of meaning attached to the verb, not as two separate ideas. So the one-word into covers movement, transformation, and these set phrases.

When it splits into two words

The two-word in to appears when in and to happen to land next to each other but belong to different parts of the sentence. Most often, in attaches to the verb as part of a phrasal verb, drop in, come in, turn in, hand in, log in, give in, and to then begins a separate phrase, usually an infinitive of purpose (to do something) or a prepositional phrase (to someone). She dropped in to say hello: dropped in is the verb, to say hello is why. He handed the report in to the manager: handed in is the verb, to the manager says where it went.

The clue is that in and to are doing separate jobs that simply collide. The in goes with the verb before it; the to goes with what comes after it. You can often feel the seam: there is a slight pause between the in and the to in speech, because they are not a single unit. If you tried to glue them into one word, the meaning would shift or break, which is the signal that they should stay apart.

The test that decides it

Try replacing the suspected into with to the inside of or, for the change sense, expressing a transformation. If that meaning fits, you want the one word into. She walked into the room means she walked to the inside of the room, so into is right. It turned into chaos expresses a transformation, so into is right.

If that does not fit, check whether in belongs to the verb and to introduces a separate purpose or destination. A reliable sub-test: can you insert the word order so that to means in order to? She came in to help means she came in in order to help, so it is two words. He turned the form in to the office: in attaches to turned (turn in) and to the office is a destination phrase, so two words. When to could be replaced by in order to, or when it starts a fresh phrase after a phrasal verb, keep in to separate.

One more giveaway: log in to a website is two words, because log in is the phrasal verb and to a website is the destination, even though log into is now common and accepted informally. The strict version splits them; the relaxed version joins them. For careful writing, the phrasal-verb-plus-to test gives you the precise answer.

Examples and a proofreading habit

One word (into): jump into the pool, fold the egg into the batter, the talks turned into an argument, I am really into jazz, she looked into the complaint, do not read too much into it. Each is movement, transformation, or a set phrase.

Two words (in to): she stopped in to check on us, he signed in to the system, please turn the keys in to reception, they gave in to the demands, I called in to report the fault. In each, in sticks to the verb and to launches a separate phrase of purpose or destination.

When you edit, every time you see into or in to, ask: is this movement or transformation (one word), or is in part of the verb with to starting a new phrase (two words)? The to-equals-in-order-to test and the phrasal-verb test resolve the tricky cases. Because both forms are valid, spell-check will not catch a wrong split, so make the test a habit. Practise on the quiz above, which mixes movement, transformation, and phrasal-verb-plus-purpose sentences, until the right spacing comes automatically.