My question is as titled. Per my understanding the "以后" in "回家以后", "从此以后", etc. is a preposition. So I am surprised the textbook says 以后 is an adverb here. Can anyone find a grammar book, dictionary, or any kind of documents to prove the textbook is correct?
I can't find a resource to cite from, but here's what I know.
In some grammar textbooks, whether Chinese or English, words like this are referred to as a Prepositional Adverb, or words that are very similar in its form to a preposition but functions as an adverb.
I am not certain about this, but some say that 以後（后） is one of the post-positions, which corresponds to prepositions in English.
Consider 以後/after in the following sentences:
- 以後(after) as a preposition.
Call me after(when) you're done.
- "After" as an adverb.
I will arrive soon after.
- 以後 as an adverb. (?)
In the future, I will call you again.
Chinese has a number of post-positions （后置介词）, instead of pre-position in English. But I have not seen an expert using this term.
A preposition or post-position （介词） is a word that requests a noun （介词宾语） to form a phrase （介词短语）. This phrase could be used as an adverb （介词短语作状语）.
(1) 他们是工厂的工人. They are workers of the factory.
(2) 桌子上有一本书. There is a book on the table.
In English, for the phrase "of the factory", "of" is positioned before "the factory"; therefore, it is called a preposition.
In Chinese, for the phrase "工厂的", "的" is positioned after "工厂"; therefore, it should be, I think, called a post-position.
In your question, for the phrase "回家以后", "回家" is a noun, which is formed from the verb "回" and its object "家". "以后" is a post-position. It is not an adverb here because it requests the noun "回家".
"以后” can be used as an adverb by itself. For example, 我以后会还给你钱. I will give you money back later.
There is another example in which "然后" is used as a conjunction to connect two sentences:
These are just my two cents.
I think "after returning home" is an adverb phrase. "After" is the preposition as you said.