'When did you last see him?'
In the above sentence, is "when" an adverb? If so, what word is it modifying?
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McCawley, in TSPE, offers a theory of adverbs in which adverbs differ in type according to the category of the phrase they modify. In his theory, time adverbs, like "when" in your example, are V'-modifying adverbs, which can sometimes be promoted to S-modifying adverbs. (With the possible exception of degree adverbs, clausal adverbs don't modify words, but rather phrases.)
Without "when", your example "When did you last see him?" has the structure:
[S you [V' did [V' last see him] ] ]
Since "when" queries the time of "did", I suppose it would modify the V' with head "did", which is the topmost V':
[S you [V' [V' did [V' last see him] ] when ] ]
When an adverb?
In dictionaries question words are traditionally labelled as adverbs. I find it more practical to implement the traditional list of word classes with some additional terms just for private use. For me "who, what, why, when etc" are question words and I don't have to rack my brains what "when" modifies. Actually it modifies nothing. Its function is to introduce a question and it asks after an indication of time in a sentence of statement. With the lop-sided label adverb one gets on a wrong track in my view.
Sometimes with interrogatory sentences, the best thing to do is rearrange it into something declaratory. In this way, it is often easier to understand the sentence structure.
You did last see him when.
In this way, it is much easier to see that when is an adverb modifying "see". It's also easier to make out the subject and other pieces of sentence structure.