What is the part of speech for “where” in these two cases?

Question

I'm aware that "where" is usually an adverb, but I'm a bit confused as to cases like this:

This is where I went to school.

Is it still an adverb here? Why or why not?

Thanks in advance.

Answer 1

(1) This is where I went to school.

(1) means (2):

(2) This is the place where I went to school.

Since where is a relative word in (2), where in (2) can be thought of as a fused relative word.

Traditional Grammar Analysis

Where is an adverb. So, (2)'s where is a relative adverb. But classifying (1)'s where as an adverb is problematic, as you seem to have noticed.

Modern Grammar Analysis

Modern grammars such as The Cambridge Grammar of The English Language or Oxford Modern English Grammar classify where not as an adverb but as an intransitive preposition, which doesn't take any complement. Therefore, where in both examples above is a relative preposition.

Answer 2

In support of traditional grammar, and the answer that your examiner will want, where (at which place), there (at that place), and here (at this place) are all adverbs.

In your example, you are using the verb “to be”. In Old English, there were at least three verbs whose meanings are now encompassed by the verb “to be”[1].

One meaning is simply as a linking verb, the equality “to be”, without any semantic content, e.g. “He is tall” and means “to equal”. (Compare “Two and two is four” and “Two and two equals four”)

The other main meaning is “the existential ‘to be’” approximately equal to “to exist”, e.g. “There are lions in Africa”. The meaning of “there” has weakened – it is no longer locative - it is existential and merely confirms the existence of lions in Africa and the adverbial nature of “there” is clear.

Thus in your sentence, you are using the existential “is”:

This is where I went to school. – This [place] exists at which place I went to school.” As you know {preposition + noun/noun phrase] = prepositional modifier. “at that place” is a modifier – adverbial and modifying “exists”.

[1] note the difference with a dynamic verb "Where did he jump?" = at which place did he jump?