Sentence Correction

I came across this sentence in Princeton's SC section...

An adverb is a word that modifies a verb, adjective, or adverb.

Given that the words 'adjective' and 'adverb' take on the article 'an', is it not required to have 'an' too in the sentence? ('a' suffices for the verb)

Can anybody please explain?

You're asking a question about parallelism in lists. This comes up on the GMAT quite often.

Here's a correct sentence:

At the zoo, I saw a giraffe, a zebra, and an elephant eating lunch.

Here is another correct sentence:

At the zoo, I saw a giraffe, zebra, and elephant eating lunch.

In the first sentence, I put an article next to every item in the list. In the second sentence, I put an article in front of the first item only. Either one is acceptable.

The same is true for auxiliary verbs attaching to several main verbs in a list:

At the game, the opening pitch was thrown, caught, and given to fan.
At the game, the opening pitch was thrown, was caught, and was given to a fan.