"Into" = movement toward the inside or transformation; "in to" = when "in" is part of a phrasal verb and "to" belongs to the following verb.
Preposition of movement or transformation. Indicates movement toward the inside of something, a change of state, or division into parts. Test: can be replaced by "toward the inside of".
She walked into the room and immediately noticed something was different.
The caterpillar transforms into a butterfly during its metamorphosis.
He ran into his old professor at the grocery store on Saturday.
The adverb "in" is part of a phrasal verb (turn in, log in, hand in, give in, tune in) and "to" is the preposition of the next verb or an infinitive. "In" and "to" never form a semantic unit here.
Please come in to sign the final documents before you leave.
She turned her essay in to the professor five minutes before the deadline.
We logged in to check our account balances and found a suspicious transaction.
Common mistake
The two-step test: (1) Try moving "in" to the end of the sentence. If it works, use two words. "She turned in her essay" = "She turned her essay in" (works). So "turned in to the professor". (2) If the verb does not work without "to", try "toward the inside of". "She walked into the room" = "She walked toward the inside of the room" (works). So use "into" (one word).
Trick to remember it
The "in" movement test: if you can move "in" to the end of the sentence (or separate it from "to"), use TWO words. "She turned her assignment in to the teacher." If you cannot separate "in" because it forms part of a directional preposition, use ONE word: "She walked into the office." Another test: replace with "toward the inside of" — if it works, use "into".
Quick rule
Into (one word) = preposition of movement or transformation (replaceable by "toward the inside of"). In to (two words) = "in" is part of a phrasal verb and "to" belongs to the next verb.
Did you know?
The same principle applies to: "onto/on to". "Onto" is a directional preposition ("she climbed onto the roof"), while "on to" occurs when "on" is part of a phrasal verb ("hold on to your dreams", "pass it on to the next person"). If you can move "on" to the end of the sentence, use two words. The same test works for both pairs.
Exercise 1 · Pick the right one
She dived _____ the pool and swam to the other end without stopping.
Exercise 2 · Pick the right one
After gathering all the feedback, she handed her proposal _____ the committee for review.
Exercise 3 · Form the word
Into (one word) = preposition of movement or transformation (replaceable by "toward the inside of"). In to (two words) = "in" is part of a phrasal verb and "to" belongs to the next verb.
The two-step test: (1) Try moving "in" to the end of the sentence. If it works, use two words. "She turned in her essay" = "She turned her essay in" (works). So "turned in to the professor". (2) If the verb does not work without "to", try "toward the inside of". "She walked into the room" = "She walked toward the inside of the room" (works). So use "into" (one word).
The "in" movement test: if you can move "in" to the end of the sentence (or separate it from "to"), use TWO words. "She turned her assignment in to the teacher." If you cannot separate "in" because it forms part of a directional preposition, use ONE word: "She walked into the office." Another test: replace with "toward the inside of" — if it works, use "into".
Bryan López
Founder of SpeakUP Academy