Use into — one word — when you mean movement toward the inside of something or a change from one state to another: "She walked into the office"; "The tadpole grew into a frog." Use in to — two words — when the in belongs to the verb in front of it and the to is a separate word, usually the "to" of an infinitive or a small preposition pointing at what comes next: "He dropped in to say hello"; "She handed her notice in to the boss." Quick test: if you can slip the words in order in front of to, keep them apart.
Into is a single preposition. It ties a verb of movement or change to a destination or a result: you go into a building, you translate a poem into Spanish, water freezes into ice. One idea, one word.
In to is not a preposition at all. It is two ordinary words that happen to land side by side. Nearly always the in is doing a job with the verb before it — as a particle in phrasal verbs like hand in, drop in, give in and log in — while the to is doing a separate job: either starting an infinitive (to say, to help, to check) or acting as a tiny preposition before the next noun. Because the two words serve different masters, they stay apart. In speech the two spellings sound identical, which is exactly why the slip is so easy to make; the difference only ever shows up in writing.
Here in completes a phrasal verb — hand in, turn in, give in, cave in — and to points at a person or thing: "Turn your keys in to reception." "The council caved in to pressure." "Please don't give in to him."
Here in is an adverb of place and to means "in order to": "She stopped in to grab a coffee." "We came in to warm up." "He signed in to vote."
Test one — does "in" belong to the verb? If dropping in changes the verb's meaning — hand versus hand in, give versus give in, log versus log in — then in is glued to the verb and to is a separate word. Write two words.
Test two — the "in order to" trick. Try slipping in order in front of to. "He dropped in [in order] to say hello" still works, so it is in to. Now try a true into sentence: "She fell in order to the pool" is nonsense, so it must be the single word into.
A handful of phrases trip people up again and again. Here is how careful writers handle them.
When something changes, use one word: "The frog turned into a prince." When turn in means to hand over, use two: "He turned his lost wallet in to the front desk." And a driver "turns into" a car park — that is entering, so it is one word.
Careful writers prefer "log in to your account," because log in is the verb and to introduces the destination. (The noun is one word: "your login details.") But "log into" is now extremely common and broadly accepted, so few editors will fight it. The same applies to "sign in to." In formal writing, the two-word form is the safer bet.
"Tune in to the show" is strictly two words, though "tune into" is widespread and widely accepted. "Drop into the shop" means enter it; "drop in to say hi" means call by for a purpose. Use "check into" when it means investigate — "Let me check into that" — and either form for arriving at a hotel.
"Thieves broke into the shop" is forced entry — one word. "The firefighter broke in to rescue the child" is break in plus a purpose — two words. The spelling follows the meaning.
Exactly the same logic applies. "The cat climbed onto the roof" uses one word for a destination, but "Let's move on to the next point" and "Hold on to the railing" keep them apart, because on belongs to the verb. Master into vs in to and you get onto vs on to for free.
Refreshingly little. Unlike many usage splits, British and American English treat into and in to the same way, and every test above works on both sides of the Atlantic. American style guides such as AP are perhaps the most vocal about keeping tune in to and log in to as two words, but careful British editors say the same. The only real variation is how relaxed a publication is about the casual one-word forms log into and tune into — and that is a matter of house style, not nationality.
Careful writers prefer 'log in to,' because 'log in' is the verb and 'to' introduces what follows; the one-word 'login' is the noun, as in 'your login details.' That said, 'log into' is now common and widely accepted, so most editors allow it. In formal writing, 'log in to' is the safer choice.
'Give in to' is the correct form: 'give in' means to surrender, and 'to' is a separate preposition pointing at what you yield to. 'Don't give in to temptation' is right; 'give into temptation' is a very common error. You would only write 'give into' if something were physically going into a container.
Try putting 'in order' before 'to.' If the sentence still reads well, use two words, because 'to' is starting an infinitive: 'She came in [in order] to help.' If 'in order' makes nonsense, you want the single preposition 'into': 'She walked into the hall.' Also ask whether 'in' belongs to the verb, as in 'hand in.'
It depends on the meaning. When something changes form, use one word: 'The caterpillar turned into a butterfly.' When 'turn in' means to submit or hand over, and 'to' points at a recipient, use two words: 'He turned his report in to the manager.' And a car 'turns into' a driveway, meaning it enters.
Yes. Ending a sentence with a preposition such as 'into' is perfectly grammatical — the old rule against it is a myth, not real grammar: 'That's the mess we've got ourselves into.' You could recast it as 'into which we have got ourselves,' but that sounds stilted. Trust your ear; the natural version is correct.
