Consider the phrase go home. What part of speech is home? I'm going to suggest, following the analysis of Huddleston and Pullum in the Cambridge Grammar of the English Language, that it is an example of something that traditional grammars do not recognize.

As a precursor, I will note that "part of speech" is often much more grammatical than lexical. I mean that, for many words, what part of speech they are depends on how they're used in a phrase more than it does what word they are. The word jump, for example, is both a noun and a verb; reddened is both a verb (she reddened) and an adjective (his skin became somewhat reddened). This is in part because English has a number of zero-derivative rules that turn one part of speech into another without changing it (e.g. a verb jump can become a noun jump, meaning "an instance of jumping"). In part, it's because English has relatively little in the way of grammatical endings or agreement, and relies strongly on word order to build phrases, so it is relatively easy to drop one part of speech in a slot meant for another, without changing it, and subsequently re-interpret it as an example of the part of speech for the slot it's in (e.g., The company's in the red, where the adjective red has been turned into a noun by dropping it into a noun slot.)

All this is to say that the fact that home is usually a noun, my beautiful home in the suburbs, should not overly concern us when looking at go home.

Because, in fact, home is clearly not a noun in go home:

• *go the home
• *go my beautiful home in the suburbs
• *go house
• *go bank

That is, home is not the head of a noun phrase, because you can't put things that go around a noun in a noun phrase around it, like articles and adjectives, and you can't substitute other similar nouns for it.

It's also not an adverb. It doesn't tell us when (go tomorrow) or how (go quietly). It's not modifying the whole sentence (suddenly, she went). It's not a modifier of degree (go particularly quickly), although it's not in a context where degree modification is particularly available.

Instead, let's look at what role it's playing in the sentence. It's indicating the destination or direction of the action of go. In fact, "go + complement indicating destination or direction of going" is one of the primary uses of go. We can say with a fair amount of confidence that the role that home is playing is the complement of go.

Now, we look at what form that complement can take. It can be a prepositional phrase: go to the store, go around the bend in the river. It can be other things that look like home: go right, go south, go clockwise.

I draw three distinctions here:

1. right is used in the sense of anti-left, not in the sense of directly, e.g. not go right home.
2. I exclude go+X in the sense of become X, e.g. go mad.
3. I also exclude go+X+-ing, e.g. go shopping where the sense is closer to the periphrastic future (I'm going to learn more about Cuba) than it is to the direction/destination sense of go.

So home is a complement of go in a sense that takes a prepositional phrase as complement. Huddleston and Pullum's analysis (drawing on earlier work) is to say that home is a preposition, and that therefore all complements of go in the direction/destination sense are prepositions.

Huddleston and Pullum martial a large and convincing set of arguments that this sort of analysis is useful. For instance, it seems to immediately simplify analysis if around is the same part of speech in hang around as it is in hang around the store.

This is fairly radical, because the first thing you notice about home is that it doesn't take a noun phrase as a complement the way all traditional prepositions do (to the store). Obviously, the traditional definition of preposition is inadequate.

I can't quote Huddleston/Pullum's definition of preposition per se, but I can say that they reorganize the category of preposition by saying that a preposition licenses a complement in the same way that a verb does--i.e., that some prepositions have no complement, like an intransitive verb; some prepositions have a noun phrase complement, like a transitive verb; and some prepositions have other complements like other prepositional phrases (go around to the back) or clauses (go after the rain stops falling). And, like verbs, a preposition may be able to license different kinds of complement. after, for example, can take a complement that is null (I'll visit you after; cf. I'll come with, which is a complementless use of with that is not universally accepted), a noun phrase (I'll come after happy hour), a non-finite verb phrase (I'll come after finishing work), or a full clause (I'll come after it stops raining).

The identification of home as a preposition implies that directions like right and south are also prepositions. I am, incidentally, somewhat conflicted about clockwise, which seems to have a manner sense going along with its direction sense: How are you going to go? Clockwise versus *How are you going to go? Right; compare to Which way are you going to go? Clockwise/Right. But go clockwise around the bush/go right around the bush, where right can have its anti-left sense (as well as its directly sense); it looks like clockwise/right are the complements of go with around the bush modifying them. So probably clockwise is a preposition.

Anyway, there you go. home can be a preposition. You can go home now.