Are you going to check this out? Yes, I will look into/in to this. Which is correct?
Into.
“Into” is much more common. Probably the only cases where “in to” would be correct are when “in” is the particle of a verb:
“He looked in to see what was in the house.”
“She stayed in to be sure she’d be home when he visited.”
That’s pretty much it. A simple test: when you say it out loud, are in and to two separate sounds? Would it sound right to put a pause between them? Most people who have grown up speaking a language have an ear for what sounds right… they just need to put in in writing the right way.
Other than “in” preceding infinitives, the only times I would see “in to” as a two-word construction (rather than using into) is in idiomatic forms including “in” like “to lead in” or “to stand in” – and even there “into” is sometimes preferable.
“Into” implies motion. If the “in” implies fixed location and the “to” motion to an idiomatic or particle use, keep them separate; otherwise usually prefer “into” over the separated form.
I came across a good example of this in an article I was editing the other day. The author had written “the plane was coming into land”. I hope that the plane was actually coming in to land; the alternative sounds rather terminal.
I like that pun… terminal, as in final, as opposed to terminal, as in where an airplane “parks”… it’s too early for me to make a punny remark to your pun, so I’ll just come out and say I like it…
The one I dislike is “onto”, even though it is in dictionaries. It just feels incorrect, somehow. I see that it is much more recent than “into”. To me it’s almost as bad as “alot”, “nevermind” or “everyday” (when the adverbial phrase “every day” is intended).