I know a farmhouse {in which | where} we can spend the night.
Where is an adverb of place, modifying can spend. But it also introduces the subordinate clause, as the relative pronoun which does. Hence where is called a relative adverb.
195. The principal relative adverbs are:—where, whence, whither, wherever, when, whenever, while, as, how, why, before, after, till, until, since.
Because of their similarity to conjunctions, these words are often called conjunctive adverbs.
- He had a fever when he was in Spain.
- Work while it is day.
- As the ship passed, we observed that her decks were crowded with Malays. [Time.]
- Keep to the right, as the law directs. [Manner.]
- You started before I was ready.
- Wait until the car stops.
- Since you came, it has rained constantly.
As and since in the sense of “because,” and while in the sense of “although,” are classed as conjunctions ([§ 368]).
The clauses introduced by relative adverbs may be either adjective or adverbial ([§§ 49–50], [379–382]).
Note. In “The more you waste, the sooner you will want” (and similar sentences) the is not an article, but an old case-form of the pronoun that, used as an adverb of degree. We may expand the sentence as follows: “To what extent you waste more, to that extent you will want sooner.” Thus it appears that the first the has a relative force, and the second the a demonstrative force.
196. An interrogative adverb introduces a question.
Where, when, whence, whither, how, why, may be used as interrogative adverbs. Thus,—