Let’s see what we can say about one of the knottier knots that tie modern English grammar together: when do we combine the words in and to and write into? Which of these, for example, is the better choice: never give into temptation or never give in to temptation?
The words in and to can each work as a preposition or an adverb, so let’s begin by recalling what these two parts of speech do and how they are composed. A preposition begins a phrase (which is a group of words without a subject and verb) that will end with a noun or pronoun. If I tell you that I would like to take a walk to the lake, the phrase to the lake begins with the preposition to and concludes with the noun lake. That noun lake is called the object of the preposition, and this group of words beginning with a preposition and ending with a prepositional object is called a prepositional phrase. There are many different kinds of phrases, and they are all named by the kind of word with which they begin. Thus there are prepositional phrases, infinitive phrases, participial phrases, and on and on. This is English grammar, after all.
The trick to remember is that a prepositional phrase in its entirety will act as a part of speech, either adjective or adverb. When I say that I would like to take a walk to the lake, the prepositional phrase (not just the preposition) is telling you where I would like to walk. Anything that answers the question where? is an adverb, and so the words to the lake form an adverbial phrase. Conversely, if I tell you that the water at the lake was cold, I’m identifying the water that I’m asserting was cold; thus this prepositional phrase is acting as an adjective, because adjectives specify nouns, making their reference more exact by description. Yes, this adjectival phrase looks an awful lot like an adverb, but the descriptive function ultimately wins out: it’s the lake water that was cold.
If we contrast now a preposition with an adverb, the adverb never has an object. There is no such thing as an object of an adverb. When I got back from the lake and walked up to the door of my house, I walked in. The word in is working with the verb to tell you where I walked, and so it functions nicely as an adverb all by itself. Had I told you that I walked into my house, I would have been combining the adverb in with the preposition to, thereby making the preposition into, which shows motion over a threshold, that is to say, movement from outside inside. This new prepositional phrase (into with its object house) would have been the adverb telling you where I had walked. To summarize, then, a preposition must have an object, and the two form a phrase that can act as an adjective or an adverb. An adverb never has an object, and works alone.
Now let’s apply this quick review to the question at hand. There is a category of verbs in English called phrasal verbs. These are formed by taking an existing transitive verb (a verb that has a direct object) and adding an adverb to it in order to form a new verb with a little twist in the meaning. If I give in to temptation, I’m not really saying that I’m giving anything to temptation, but rather that I’m yielding or submitting to it; if I am giving anything, I’m giving myself to temptation, which shows that the preposition to should not be combined with the adverb in: the word in has to remain separate in order to preserve the phrasal verb give in.
But that’s not usually the case when a verb (phrasal or not) is a verb of motion. If I discover when I return from my walk that someone broke into my house, I would be saying first what someone did—entered or trespassed, and then where they did it—into my house. The preposition into (in + to) is correct here because there is indeed motion over a threshold—namely, my front door. One roughly (I say again, roughly) correct, rule-of-thumb test to help decide whether to combine in with to is this: if you find yourself accenting the adverb (don’t give ín to temptation), you probably have a phrasal verb, in which case you keep in and to separate. If instead you accent the verb (I wálked into the garden), combine the two words to form the preposition into.
As you can probably imagine, there’s more to this knotty problem than the few strands we’ve pulled apart here, but what we have learned will get us looking in the right direction to understand it better. What makes this a difficult subject is also something we should always remember about writing: the written word comes after the spoken word. We humans first speak, not write—that adorable newborn we hold in our arms will one day gurgle and burble, not write, back to us—and what we do eventually write is an attempt to transcribe that ever-flowing mental life of our human experience. In writing we become more conscious, more aware of our experience, but in doing so we also stand further from it. That’s where confusion can arise, and sometimes we can work out a compositional problem by speaking a phrase or sentence aloud, the better to return to the living moment that gave it rise.