in, into

 , in to

 Generally, in indicates a fixed position ("He was in the house") while into indicates movement toward a fixed position ("He went into the house"). There are, however, many exceptions (e.g., "He put the money in his pocket"). As so often with idiom, there is no describable pattern to these exceptions; it is just the way it is.

 Whether to write into as one word or two also sometimes causes problems. The simple rule is that in to is correct when in is an adverb, but the distinction can perhaps best be seen in paired examples: "He turned himself into [one word] an accomplished artist" but "The criminal turned himself in to [two words] the police."