After seeing into all my life and thinking nothing of it, over just the past few months, it has caused me an existential crisis.
My only personal run-in was with some high school short story I wrote where a character walked “into the door” somewhere. My English teacher said it meant the character had actual impact with the door.
Yikes! Lesson learned.
(On the other hand, if I’d written that he walked “through the door,” this teacher, who seemed to take everything literally, would have claimed the character reduced the door to splinters.)
Now it seems everybody uses into as one word EVERY TIME.
I consulted Grammar Girl™ Mignon Fogarty’s Quick and Dirty Tips™, and found some comfort and clarification. She explained that into is a preposition generally relating to direction. (He walked into the room.)
I’ll add that it also relates to transformation. (She turned into a witch for Halloween.)
And I’ll add that it may be one word if you mean “to go inside” or “within.” (He jumped into his jeep.)
One more addition: It’s one word if you mean “intense interest.” (He’s really into playing with model trains.)
I’ve seen the following example written as one word, but I would say it should be two, like this:
I’m going to move in to a new house.
That’s because we call it “move-in day,” not “move-into day.” And because the sentence is intended to convey the act of moving rather than that the writer is going inside a new house.
Here’s an example as written that made my eyelid twitch because I think it’s wrong:
I can’t come into work on weekends.
The author meant that he couldn’t go there to do any work, not that he was physically unable get inside the office.
In my book, these next examples are also two words, even though you could make a weak argument that some direction is implied.
The maid came in to tidy up.
I think I’ll turn in to bed for the night.
The suspect turned himself in to police.
Bottom line: In my own writing, I find myself trying to avoid using into altogether. It’s not easy. There has to be a better way.