If you want it back ...
I'm doing a school project and need to figure out parts of speech in my letter that I wrote, but, I dont know what "back" is, can anyone help?
This is a good question. Traditional nineteenth century grammarians and most twentieth century ones classified it as an adverb. The reason is that it did not fit their definitions of nouns, verbs, adjectives or prepositions.
However, many twentieth century grammarians since the 1920s, realised that back and similar words have none of the properties of adverbs and nearly all the properties of traditional prepositions. They recognised that the traditional word class preposition was badly conceived and needed to be rethought, because these types of words, like back, away, out and so forth were clearly the same types of word as in, on and at. They also recognised that the name for this category of word, preposition, is a bad one. However, they thought it would be too difficult to change.
Most serious twenty-first century grammarians and syntacticians now recognise words like back and away to be intransitive prepositions. They are described as intransitive because they take no 'object'.
These are the ways that words like back are not like adverbs:
Adverbs cannot be used as predicative complements of the verb BE:
In standard Englishes, adverbs cannot usually be modified by the specialised adverbs straight and right
Most adverbs can be modified by the adverb very.
It is extremely rare for adverb phrases to be able to modify nouns, but back-phrases can:
Adverbs cannot be used as locative complements of verbs such as put:
Now see that regular prepositions, that take noun phrase complements, pattern just like back:
Complements of BE:
Modification by straight and right:
modification by very:
Traditional twentieth century grammars (and hence most dictionaries, which are wildly out of date) regard back as an adverb. Modern academic grammars classify back as a preposition. Unless the Original Poster has a lot of time to explain why back is a preposition, they might be better off describing it as an adverb in their project.
Alternatively, they could cite a modern academic reference grammar such as: