He arrived moments later.
Here moments is noun and clearly later must be adverb, then how is it modifying the noun?
He arrived moments later.
Here moments is noun and clearly later must be adverb, then how is it modifying the noun?
Actually, you've got it backward: moments modifies later.
How much later? —moments later.
More commonly we see expressions like moments (CGEL calls them measure phrase modifiers) with a specific quantifier—some moments later, three hours later ; the bare plural noun leaves the exact quantity vague but in effect provides a scale by which the lateness may be measured.
This illustrates an important point: part-of-speech (or as we now say, 'word class') has only limited usefulness for describing how a particular word is used in a particular context. We are taught when we're just beginning grammatical analysis that words 'belong' in a particular category which dictates how they can be used; but eventually we reach a point where the categories break down. I'm sure you're familiar with "verbs" being used as "nouns" (Playing video games is fun) "nouns" being used as "adjectives" (street corner, office manager) and "adjectives" being used as "verbs" (First you have to brown the meat); here you have a noun used as an adverb. English does this sort of thing all the time.