Read about the case of the preposition/adverb conundrum.
It may seem like a no-brainer to label all prepositions as adverbs when they modify verbs (again, that’s in those cases when they have no object). Indeed, decisions like this are a common problem to solve in related fields like computational linguistics. Computational linguistics is a science that does things like create software to do language-related tasks, such as translate written language, program voice recognition software for when you call an automated 800 number, or tag (which means “label”) the parts of speech of written words. For example, here in an early computational linguistics article, the tagging software that the authors describe had an extra portion of code to teach the computer when to call as a preposition, and when to call as an adverb. For example, in this next sentence, the first time we use the word as, it’s an adverb, and the second time, it's a preposition:
- The Catskill mountains are not as (adverb) tall as (preposition) the Himalayan mountains.
The first as is an adverb because it modifies the adjective tall, but the second as is a preposition because it acts as a comparison, and the noun phrase the Himalayan mountains is arguably the object of the preposition.
The Adverb/Preposition Distinction Is Not Straightforward
Now, let’s go over some fascinating theoretical arguments for why “adverb” may not always be the best label for those prepositions with no object. Even syntacticians disagree, but their reasons are interesting.
Adverbs tend to be considered grammatically optional.
First, adverbs tend to be considered grammatically optional. In other words, they add detail and meaning to the sentence, but don’t cause the sentence to be wrong if they’re removed. “Jennifer runs fast” and “Jennifer runs,” without the adverb fast, are both grammatically correct. Prepositions, on the other hand, play a much more important role in the grammar of their sentences. In that way, we could argue that in “Get inside!”, inside is not grammatically optional (because “Get!” alone means something else, if anything). This makes inside a lot more like a preposition than an adverb, even though some grammarians would call it an adverb.
A second argument in favor of the preposition label is the fact that the sentence “Get inside!” may be a case of what linguists call “ellipsis.” (It’s not the same as the punctuation, but it’s a similar concept: Think of the way the three-dot punctuation mark is used to show a trail-off of missing words.) Speakers frequently “elide” words, which means that they are not spoken, but they are implied. We know they are there, but it may be redundant to say them out loud. In other words, if I point to the castle and shout “Get inside!” some linguists might argue that inside does indeed have an object—the castle—it’s just that this object is provided by context, and is not needed in the grammar of the sentence. The linguistic study of when context interacts with syntax is called “pragmatics.”
A third pro-preposition point is that these controversial adverbs fail certain syntactic adverb tests. For example, take the prepositions inside, up, around, and over. We know that adverbs can modify adjectives, like incredibly in “She is incredibly smart” (incredibly is an adverb modifying the adjective smart) and they can modify other adverbs, like incredibly in “She drove incredibly fast.” (Incredibly is an adverb modifying the other adverb fast.) However, the preposition around is different: We can’t say “She is around smart,” and if we say “She drove around fast,” around is not modifying the adverb fast. The preposition inside has similar problems: “She is inside smart” and “She is inside fast,” show that inside fails the adverb tests, making many of us inclined to call it a preposition no matter what job it does. (2) This shows that while incredibly is an “indisputable” adverb, prepositions like inside and around when they have no object fall further away on the spectrum of adverbs, so saying that inside is an adverb just because it has no object may not be 100% right. Even though not all adverbs pass all syntactic adverb tests, some members are more controversial than others. Remember that those “indisputable” “-ly” adverb members, like incredibly, on the other hand, are not controversial! We’re all in agreement about those.