Quick answer
Into is a single preposition showing movement or transformation — walked into the room, turned into a frog. In to is two separate words that happen to sit together, usually where "in" belongs to a verb — she gave in to the pressure, log in to the site.
The core difference
A preposition, and a coincidence of two words meeting.
- into — movement inward or transformation: drove into town, translated into French.
- in to — "in" belongs to the preceding verb: gave in to, turned it in to the office, log in to.
The test
Ask whether in belongs to the verb. "Give in", "turn in", "log in", "check in" are phrasal verbs — the in is part of them, so to stands separately. If the verb works without in, you want into.
The one that changes meaning
"She turned herself into the police" and "she turned herself in to the police" mean startlingly different things. The first is a transformation; the second is a surrender. That is the clearest illustration of why the space matters.
Frequently asked questions
Is it "log into" or "log in to"?
Log in to. "Log in" is a phrasal verb, so the "to" stands separately.
What is the test?
Ask whether "in" belongs to the verb. If it is part of a phrasal verb like give in or turn in, keep the space.
Does the space ever change the meaning?
Yes — "turned herself into the police" is a transformation; "turned herself in to the police" is a surrender.
