>>716500910it is typically taught that you use "a" before words that start with a consonant and "an" before words that start with a vowels.
however, this isn't actually the rule, just the norm. the purpose of "an" is to make saying the next word easier, and as such, it is grammatically correct to use either "a" or "an" in _all_ circumstances.
"herb" has a silent "h" as pronounced, so phonetically it starts with a vowel, and as such using "an" fits the normal pattern better than not.
i've met foreigner nationals with better english skills than most americans at uni, even though it's not their primary language.
in other words, you're almost certainly a retarded uneducated american