...with examples from Stargate Atlantis, Sherlock Holmes, Psych and House...
starwatcher307 asks: When do you use into versus in to?
Using into and in to can be confusing since they look nearly identical, with one being a single word and the other being two. They both have different meanings, however.
The word into is a preposition that is used to indicate movement, often toward the inside of a place, though not exclusively. It can also indicate such things as the condition, state, or form of or a continuing extent in time or space.
McKay accidentally drove the jumper into a wall of the jumper bay his first time at the helm.
Hearing the frantic alarms, Foreman and Thirteen rushed into the patient's room.
Holmes stared at the cocoon for days, unwavering, before the captured caterpillar finally changed into a butterfly.
The stake-out lasted well into the night.
There are about a dozen different uses for into, so be sure to consult a dictionary when unsure of a specific usage.
To the contrary, in to is an adverb followed by a preposition or the first word of an infinitive and has to do with both direction and purpose.
The offworld team knew Weir wouldn't give in to the demands of their captors.
Lassiter turned his report in to the chief.
Shawn went in to buy pineapple juice.
Eyeing the scene of the crimes, Holmes quickly stepped in to search for clues.
In the examples above, to the demands of their captors and to the chief are prepositional phrases, while to buy and to search are infinitives. If you remove the to from any of these, they, obviously, cease to be prepositional phrases and infinitives. Here, the in and to serve two different functions, so they can't be replaced by the single-functioning into. Thus, you should always use in to in such instances.
Summary
When writing, look for prepositional phrases and infinitives. If you have any, you will likely need to use in to to keep them intact. If you are moving to someplace, into is likely to be what you need. If all else fails, take a good look at your sentence to see if it actually makes sense... Can Lassiter really turn his report into the chief?!
Resources:
The Blue Book of Grammar and PunctuationDictionary.com On Target: A Web Newsletter with Communication TipsWord-Mart