We all know this English rule: Use an before vowel sounds...
An IPO
An amah
An uncle
and a before consonant sounds.
A cheongsam
A sampan
A tram
A common mistake English learners make is to just check the first letter, not the first sound.
Some words start with a consonant, but have vowel sounds.
An MTR station
An hour
An honest man
Some words start with a vowel, but have consonant sounds.
A European
A university
A one-year old
That is simple and easy to understand. Then like many things in English there are exceptions.
What about the word herb? British usually pronounce the h and Americans often don't. What happens is that even people who pronounce the h often use an.
There is also an historic exception for the word historic. Some English speakers used to drop the h when saying words like historic, humble and hypothesis, but don't do it anymore. However they often keep on using an.
Even if it sounds strange to native speakers it is much easier to ignore the exceptions. English is an international language and second language learners outnumber the native speakers. Feel free to say, "Rosemary is a herb."
by John Larrysson
A native English speaker who has been teaching practical English in Hong Kong for more than a decade.