It's an interesting problem. The thing to remember, though, is that articles aren't independent words. They are used only as determiners of nouns, and fulfil no grammatical role otherwise. There are various examples in the the comments, such as
This is merely an example
And the question is, what does merely modify. Actually, what it appears to modify is a noun phrase, "an example", but that would be strange - if it can't modify a noun, surely it can't modify a noun phrase?
What you're actually seeing here is an area that linguists get into arguments about. Sometimes you see an adverb that doesn't appear to be modifying an identifiable verb or adjective. Linguists then tie themselves in knots trying to come up with an explanation that doesn't contradict the nice, simple basic explanations people get taught in school.
Sometimes, it's easy. Sometimes, it's an adverb modifying an entire clause:
Suddenly, a voice cried out.
The adverb is prepended to the clause and modifies the whole thing, with a subtly different meaning to "a voice suddenly cried out". Such whole-clause adverbs can also be appended, placed after the clause. They don't generally apply to the whole clause when placed in the middle of it, though.
But it's not modifying is. It's unusual, but you can say:
This merely is an example.
However, then you're not saying that all it is is an example, as you are with the merely after is. You're insisting that it is an example, a difference that is clearer if you use the nearly synonymous (in this context) simply:
This is simply an example
This simply is an example
The first would be used to reassure people that this is just being used as an example, not as something that they should worry about in itself. The second would be used if someone were arguing about whether it was an example, and you were insisting - using certain sorts of argument - that it was.
So, if merely or simply appears after is, what is it modifying? Some linguists will say it's modifying a noun phrase, with the explanation that some adverbs can modify some noun phrases in some situations. Some linguists will say it is modifying the determiner, with arguments against it modifying the noun phrase being that it can't modify an arbitrary noun phrase. However, it's not always as simple as that. The meaning of the adverb depends on what the determiner is. For example, consider:
This is merely an example.
This is merely one example.
The first is saying that it is an example, and shouldn't be taken as having any other significance. The second is asserting that there are other examples out there that could be chosen. In the second, it is clear that the adverb is modifying the determiner one. In the first, it is the "example-ness" that is being modified.
It's not hard to see how linguists end up in knots over this stuff. If you work at it, you can find examples of adverbs applying to pretty much any part of speech, and then you argue about how it should be parsed. The bottom line, really, is that different adverbs can apply to different things in different situations, and the important thing is to learn how they are used rather than coming up with general rules, because any general rules like that will have to be supplemented by an immense list of exceptions.