Correct usage of into and in to is as much of a challenge to native English speakers as it is to learners of the language.

What’s the Difference Between Into and In To?

“Into” is a preposition that expresses direction or movement of someone or something.

He put the key into his pocket.
 Karl walked into the dentist’s office, filled with dread.

Note that the direction is always “in”—inside, to the interior of, towards.  You can also often answer a “where” question when “into” is used.  “Into” can also denote some kind of transformation is or has taken place.

Where did he put the key? Answer: “into” his pocket, inside his pocket
Where did Karl walk? Answer: “into” the dentist’s office, to the interior of the dentist’s office.

“In to” is the adverb, “in” + the preposition “to”, or “to” as part of the infinitive.  You don’t use “in to” when you would use “into”, the 2-word phrase means something else entirely, and the words are next to each other to satisfy sentence construction.

“In” works with whatever the verb is to become part of the verb phrase.  It modifies the verb, and so is an adverb.  The way it modifies the verb is to answer “how”, “where”, “when”, “why”, or “to what extent”.

He agreed in total with the decision.
“To what extent” he agreed is answered here; he’s all in!
“To”, in the 2-word phrase “in to”, can be used as a preposition, and will take an object.
We gave in to his request.

“In” in this sentence answers a version of “where”, and “to” answers to “what”—his request.  “His request” is the object of the preposition “to”.

When the “to” of “in to” is used as part of the infinitive it will mean, “in order to”, or “so that”.

We raced in to avoid being late.
The “in” part answers the question “where”, and the “to” part answers “in order to” what?  Avoid being late.

Using the wrong one—“into” when you should use “in to”, or vice-versa can completely change the meaning of what you’re saying.

The girls all ran into the lockers.
The girls all ran in to the lockers.

One of these sentences results in girls with potential head trauma!  “In to” is the correct choice.  If they ran “into” the lockers it would mean that they were running headlong into a metal storage locker.

One trick is to try saying the sentence out loud with both into and in to —making sure to pause between “in” and “to” for the 2-word phrase.  One of the two versions will sound odd, and one will sound correct.  Listening is one of the most important aspects of learning.