Into
The preposition into has many definitions that all relate to direction. He walked into the candy cane forest, Santa logged into his email…you get the idea.
In to
The word in can be an adverb, adjective, or a preposition, and the word to by itself can be an adverb or a preposition. Sometimes these two little words simply end up next to each other.
An example would be how the elves turn their toys in to Santa for evaluation. They turn them in. To whom? Santa. (You didn't think Santa only checked his list twice, now did you?)
If the in in in to/into is part of a phrasal verb, you'll want to leave that space in there. Phrasal verbs generally have two words, and they include a verb and either a preposition or an adverb—something like dig in and give in.
With over 200—count 'em, 200—of these phrases in the English language; do not even attempt to memorize them all. Trust us. We wasted way too much time and brainpower with zero results.
Example
The woman refused to give into her parrot's squawking demand for a cracker.
The golden retriever at the dog park was able to shove seven tennis balls into his mouth.
Which sentence uses the preposition into correctly? It's the second one. Where did the dog shove the tennis balls? Into his mouth, where else? The first sentence is incorrect because there's no direction involved. It should say that the woman refused to give in to her parrot. Welcome to the wonderful world of phrasal verbs, Shmoopers.