It depends on how you think about grammar. If you like the **adverb** as a traditional part of speech, then sure, it's an adverb. If you analyze grammar and syntax based on function, then you might agree with some linguists that *home* is an **adverbial** / **prepositionless prepositional phrase**, or you might agree with other linguists that it is a **preposition**. I'll lay out each of the cases in detail.
Adverb?
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First, dictionaries will tend to classify this usage of *home* as an *adverb* because lexicographers are being asked to put each single word into a part of speech, and they aren't doing much sentence-level analysis. So they pick one of the parts of speech inherited from 18th and 19th century grammarians, "*adverb*," which is really a catch-all for words that seem to modify adjectives, verbs, clauses, prepositions, and other adverbs (ThoughtCo, "[What Is an Adverb?][1]"). So the Oxford English Dictionary has an entire entry, "[home, adv.][2]," including this entry describing the usage in "He is home":
> 1e. Without verb of motion. Arrived at one's house, neighbourhood, or country after a period of absence. Also: in one's home; at home.
And Merriam-Webster has an entire article breaking down adverbs as "[a most fascinating POS (part of speech).][3]" In this system, *home* would be an *adverb of place*. Adverbs are the catch-all category of traditional grammar.
Adverbial? Prepositionless Prepositional Phrase?
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*Home* as adverb is less satisfying to linguists who want a more nuanced description of how language functions on a syntactic level. Does *home* really work the exact same as other *adverbs of place*, which might also be considered prepositions (*on*, *around*), adjectives (*backwards*), or nouns (*everywhere*) in other contexts? More robust labeling of a word or phrase's function gets around the difficulties around parts of speech. So Rod Mitchell and other linguists can say *home* is a noun, and when it appears to be an adverb in the traditional system, it is actually a prepositional phrase (or in Rod Mitchell's words a "prepositionless prepositional phrase") where the preposition is elided. (The OED poses elision as a possibility in the etymology for "home, adv.") In this reading, the three sample sentences might be rendered like so, with parenthesis representing the elision:
> 1. He is (at) home ("home" is noun within a prepositional phrase where the preposition is elided)
>
> 2. He is at home ("at home" is a prepositional phrase denoting location)
>
> 3. He went (to) home ("to home" is a prepositional phrase denoting destination)
In this kind of treatment, the explicit or implied prepositional phrase might also be called an *adverbial phrase* ([ThoughtCo][4]), signalling that it is performing the function of modifying a verb, adjective, or clause. This kind of terminology is still generally familiar to the traditional grammar crowd, but technical enough to acknowledge how phrases and single words can both have adverbial functions.
Preposition?
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Then there are readings that insist *home* should be another part of speech altogether - a preposition. Huddleston and Pullum have argued that *home* should be considered a preposition. Pullum models a more accessible version of the argument in [Lingua Franca][5]: prepositions don't merely govern/precede nouns and pronouns; they work with clauses and even by themselves. They want the class of *prepositions* to be expanded to include these spatial words. [Maria Brenda explains in greater depth in her exhaustive book on the spatial preposition "over"][6]:
> Another group of words which do not license noun phrase complements consists of spatial terms which function as a goal complement with the verbs *come* and *go* and as a locative complement of the verb *be* (Huddleston and Pullum 2002:614). The goal complements of the verbs mentioned, such as *ashore*, *upstairs*, *home*, or *indoors*, are traditionally considred adverbs. Huddleston and Pullum (2002) argue, however, that they should be reassigned to the prepositional category. They claim that adverbs generally cannot function as goal complements to the verbs of motion and as locative complements to the verb *be*.
The idea is that other spatial adverbs don't substitute in:
> x 1 (modified) He is locally.
And also that the words like *home* are not modifying the verb, any more than *young* in
> He is young
modifies the verb. Brenda takes issue with this last line of argument, pointing out that words like *home* don't modify the subject of the sentence either, and thus it seems premature to suggest they aren't in some way modifying the verb. Whether you agree with Huddleston and Pullum or not, they show how linguists are attempting to group or classify usages like *home* according to deep syntactic functions, and how they test each other's own terminology through comparisons and exceptions.
[1]: https://www.thoughtco.com/what-is-adverb-1689070
[2]: https://oed.com/view/Entry/87872
[3]: https://www.merriam-webster.com/words-at-play/the-adverb-the-most-fascinating-pos
[4]: https://www.thoughtco.com/what-is-adverbial-grammar-1689067
[5]: https://www.chronicle.com/blogs/linguafranca/2013/02/05/being-a-preposition/
[6]: https://books.google.com/books?id=TxJQBwAAQBAJ&pg=PA89&dq=%22Huddleston%20and%20Pullum%22%20home&hl=en&sa=X&ved=2ahUKEwjq_9n18sbkAhUNheAKHQxQAngQ6AEwBHoECAQQAg#v=onepage&q=%22Huddleston%20and%20Pullum%22%20home&f=false