Into (one word) is a preposition showing movement inside something, a collision, a transformation, or deep engagement with something. In to (two words) is what you write when “in” belongs to a phrasal verb like log in, drop in, or check in, and “to” begins the phrase that follows it separately.
The most reliable test: try substituting “in order to” where in to might belong. If that substitution makes sense, use two words. If you need “inside” or “to the inside of” instead, use one word.
She dropped in to say hello. → She dropped in order to say hello. ✔ → in to
The cat jumped inside the box. ✔ → The cat jumped into the box. → into
Why These Two Get Confused
The confusion is structural, not semantic. Into and in to sound identical, so your ear gives no information. The real trap is a specific category of English verb: phrasal verbs. Verbs like log in, drop in, check in, tune in, hand in, and turn in already contain “in” as an essential particle. When “to” appears after one of these verbs, the instinct to fuse them into “into” is strong — but doing so either changes the meaning entirely or produces nonsense.
There’s also an auditory cue that most grammar guides miss: in phrasal verbs, the stress typically falls on “in” rather than on the verb — log IN, drop IN, check IN. When you hear that stress pattern, you’re dealing with a phrasal verb, and “in” needs to stay attached to it, separate from “to.”
The Four Functions of “Into”
Into does four different things in standard English, and knowing all four prevents the most common errors.
Physical movement — entering a space: The most common use. Something moves from outside a location to inside it.
She walked into the room. The dog ran into the yard. He dove into the pool.
Collision or impact: Something comes forcibly into contact with something else.
The car skidded into the barrier. He bumped into a former colleague at the airport.
Transformation or change: One thing becomes something different.
The caterpillar changed into a butterfly. Water turns into steam at 100°C. The conversation turned into an argument.
Engagement or deep interest: An informal but fully standard use expressing involvement or enthusiasm.
She’s really into hiking. He got into photography during the pandemic. I’m not into true crime podcasts.
All four of these take into as a single word. None of them involve a phrasal verb. That’s the structural key: if the verb before “in” cannot stand alone without it — if “in” is making the verb mean something different — then “in” is an adverb particle attached to the verb, and “to” belongs to the phrase that follows.
The Three-Part Test
When you’re uncertain, run three questions in order.
Question 1: Does the sentence describe movement toward an interior, a collision, a transformation, or deep engagement? → Use into.
She jumped into the car. Clay shaped into a bowl.
Question 2: Is “in” part of a phrasal verb — a verb that changes meaning when “in” is attached, like log in, hand in, drop in, check in, tune in, or turn in? → Use in to.
Please log in to your account. She handed the report in to her manager.
Question 3: Does “to” introduce an infinitive that follows independently? → Use in to.
He stopped in to pick up the package. She dropped in to say goodbye.
If none of these tests settle it cleanly, try the “in order to” substitution. If it fits, use two words. If you need “inside” instead, use one.
The Style Guide Verdict: Log In To, Not Log Into
This is the question that tech writers, UX writers, and workplace communicators search most often, and it has a definitive answer.
Both the AP Stylebook and the Chicago Manual of Style require log in to. The AP Stylebook is explicit: “Use as two words in verb form: I log in to my computer.” The Chicago Manual of Style’s Q&A states that formally correct usage calls for “log in to” — with “in to” as two words — because “in” forms part of the verb phrase, where it functions as an adverb rather than as a preposition. The same rule applies to sign in to, sign on to, log on to, and tune in to.
The reason is grammatical, not arbitrary. In log in, the word “in” is an adverb modifying “log.” It is not functioning as a preposition pointing toward an interior — it is completing the phrasal verb. Fusing it with “to” to form log into would make “into” a preposition, which changes both the word’s grammatical role and, technically, the sentence’s meaning.
Many informal sources and some technology companies use log into in casual copy. In formal editorial writing, published technical documentation, and any content following AP or Chicago style, log in to is the required form.
A Related Distinction: Login vs. Log In
The confusion often extends to the noun form. Login (one word, no space) is a noun or adjective: Enter your login credentials. The login page failed to load. As soon as the word functions as a verb, it becomes two words: Please log in to access your account. You need to log in first. This noun-verb distinction is a separate question from into vs. in to, but they appear together often enough that getting both right matters.
The “Onto/On To” Parallel
The same rule that governs into/in to applies exactly to onto/on to, which helps confirm the pattern.
She climbed onto the roof. → onto as a preposition of movement
She held on to the rail. → “hold on” is the phrasal verb; “to” follows separately
Move on to the next question. → “move on” is the phrasal verb; “to” follows separately
The dog jumped onto the couch. → physical movement upward onto a surface
If you can remember onto/on to, the into/in to rule is already in your head.
When Each Choice Sounds Wrong
Swapping the forms produces sentences that range from awkward to semantically broken.
Wrong: I dropped into say hello.
This suggests you dropped physically inside something called “say hello,” which makes no sense. The correct form is I dropped in to say hello. (“Dropped in” is the phrasal verb; “to say hello” is the reason.)
Wrong: She turned her report into the office.
This means the report transformed into an office — not the intended meaning. The correct form is She turned her report in to the office. (“Turned in” is the phrasal verb; “to the office” shows the destination.)
Wrong: Please log into your account and update your password.
Technically understood, but incorrect under AP and Chicago style. The correct form is Please log in to your account and update your password.
Wrong: The kids ran in to the pool.
Here the movement toward the interior is the meaning, and “in to” breaks the preposition. The correct form is The kids ran into the pool.
Common Mistakes and Quick Fixes
Mistake: Using into with a phrasal verb because it sounds natural.
Fix: Test whether “in” is completing the verb. If log, drop, check, tune, hand, or turn changes meaning when “in” is removed, keep “in” attached to the verb and separate from “to.”
Mistake: Writing in to when describing physical movement or transformation.
Fix: Try substituting “inside” or “toward the interior of.” If that substitution fits, use into.
Mistake: Writing login as a verb: You need to login.
Fix: As a verb, it’s always two words: You need to log in. As a noun or adjective, it’s one word: your login details, the login page.
Examples Side by Side
The dog ran into the yard. (physical movement, preposition of interior)
I stopped in to check on the dog. (phrasal verb “stopped in” + separate “to” phrase)
The water froze into ice. (transformation)
She came in to check on the temperature. (phrasal verb “came in” + infinitive)
He’s really into jazz guitar. (engagement/deep interest)
He tuned in to the jazz station every Sunday. (phrasal verb “tuned in” + destination)
Please log in to your account. (phrasal verb, formal style)
She turned the keys in to the property manager. (phrasal verb “turned in” + destination)
Quick Practice
Choose the correct form.
- He walked [into / in to] the conference room just as the meeting started.
- Please log [into / in to] the system using your company credentials.
- She came [into / in to] check on the patients every hour.
- The proposal was turned [into / in to] the committee before the deadline.
- They ran [into / in to] an old friend at the market.
Answers: 1. into (movement) — 2. in to (phrasal verb log in) — 3. in to (phrasal verb came in + infinitive “to check”) — 4. in to (phrasal verb turned in + destination) — 5. into (collision/unexpected encounter)
FAQs
What is the fastest test for into vs. in to?
Try substituting “in order to.” If that substitution makes sense in the sentence, use in to. If you need “inside” or “toward the interior of” instead, use into. For phrasal verbs specifically, ask whether “in” is part of the verb’s meaning — if removing “in” changes what the verb means, keep “in” with the verb and separate it from “to.”
Is “log into” or “log in to” correct?
Log in to is correct under formal editorial style. Both the AP Stylebook and the Chicago Manual of Style require log in to, because “log in” is a phrasal verb where “in” functions as an adverb, not as a preposition merging with “to.” The same rule applies to sign in to, sign on to, and log on to.
What is the difference between the noun “login” and the verb “log in”?
Login (one word) is a noun or adjective: Enter your login. Go to the login page. As a verb, it is always two words: Log in to your account. You need to log in first. Treating login as a verb is a separate, common error.
Does the same rule apply to “onto” and “on to”?
Yes, exactly. Onto is a preposition of movement: She stepped onto the platform. When “on” is part of a phrasal verb, it stays with the verb and “to” begins the next phrase separately: Move on to the next section. She held on to the railing. The logic is identical.
Can “into” ever describe transformation or interest — not just physical movement?
Yes. Into has four standard uses: physical movement, collision or impact, transformation (turned into a problem), and deep interest or engagement (she’s really into yoga). All four use into as a single word. None of them involve a phrasal verb, which is what distinguishes them from the in to cases.
How do I remember which phrasal verbs need “in to”?
Recognize the most common ones: log in, sign in, check in, drop in, tune in, turn in, hand in, step in. In each case, removing “in” changes the verb’s meaning significantly. Whenever you see one of these and “to” follows, write two words. You can also listen for stress: in phrasal verbs, the stress typically falls on “in” itself, which is an auditory cue that “in” is the particle, not part of a merged preposition.
The Bottom Line
Into is a single preposition showing movement, collision, transformation, or deep engagement. In to is two words that belong apart when “in” is already doing a job as part of a phrasal verb. Run the “in order to” substitution test when you’re unsure, know the most common phrasal verbs (log in, drop in, check in, tune in, turn in), and remember that both AP and Chicago style are clear: log in to is correct, log into is not. The onto/on to pair follows the same rule if you ever need a second example to confirm the pattern.
Conclusion
The choice comes down to one question: is something moving inside or changing form, or is “in” actually part of a verb that “to” just happens to follow? Run the “in order to” test whenever you’re unsure, and “into” versus “in to” stops being a guessing game.
