Take "into" and "in to," for example. Which do you use for what type of circumstance? Or are they interchangeable?
The answer is no, they are not interchangeable.
According to Quora.com, "into" is a preposition that refers to motion (he walked into the room; she climbed into the bath), whereas "in to" is a construction that arises from a phrasal verb ending with in (for example, "be in," "break in," "come in," "hand in") being followed by to as either a preposition:
- You should hand the form in to your teacher
- The plane came in to land
So, here's the skinny:
If you can replace "into"/"in to" with "out of" and the sentence still makes sense within the context of the sentence, use "into." If you can't, then the choice is "in to." Additionally, where you can break a sentence down so that it's still a complete sentence will give you a clue as to which version to use. Let's look at some examples (and I apologize up front, because I'm going to make you think with these):
He walked into the room.
I could easily say he walked out of the room, so "into" is the correct usage here. Also, "into the room" modifies the verb "walked." Consequently, we could say "He walked" by itself and it's a complete sentence, but "into the room" gives us more information about where he walked.
Janice climbed into the bath.
Again, since "Janice climbed out of the bath," makes sense, "into" is correct. As with the above sample, saying "Janice climbed" still works as a complete sentence, but "into the bath" tells us where. This is another prepositional phrase modifying the verb "climbed."
You should hand the form in to your teacher.
Obviously, "You should hand the form out of your teacher," doesn't make sense, so "in to" is the correct version to use. With this example, "You should hand the form in" is a complete sentence. It works. But let's take a look at other ways we could write this. "You should hand the form." Nope. That doesn't work. It's a fragment. "You should hand the form into." No again. This doesn't work as a complete sentence. You might be saying, "Well then add 'your teacher' and it's complete and 'into' works." The problem with that is that "your teacher" is not a prepositional phrase and can't stand as a modifier. So, "You should hand the form in" is the only functional, working version of this sentence, and "to your teacher" is the prepositional phrase that modifies the verbal phrase "hand...in"
Oh my God! That was tricky even for me to flesh out, but the point is, if you break down the sentence, you can figure out whether to use "into" or "in to." Yep, this is where all those horrific exercises in English class come in handy, where you break down a sentence into all its different parts.
Okay, so what about: "Tell him to come in."?
This is one of those blurry areas. Technically, the sentence is "Tell him to come into my office," (or my room, my house, or wherever) but that's not how we speak. So we stop at "in." This is okay. Just know that if you tack on the additional ending, you will need to use "into" and not "in to."
Keep in mind, one version of the into/in to pairing refers to motion and is a preposition, while the other is part of a verb phrase and could be modified by a prepositional phrase starting with "to." Also keep in mind where the sentence can stop and still be considered a full, complete sentence. If you stop before "into," then "into" is the right choice. If you stop at "in," then "in to" is the right choice.
I know. This is still tricky, but at least it's a start to understanding this confusing word pairing.
Happy Writing and Reading!