grammar - Article when there is an adjective before a noun

When to use a or an before a noun when there are adjectives before that noun?

like the following example:

An operator pressed the button.

should that make:

A professional operator pressed the button.

An professional operator pressed the button.

same for:

A Hypertext markup language.

An Hypertext markup language.

Sometimes it seems a bit confusing, especially when there are more adjectives like:

An omen was revealed to the priest.

A terrible omen was revealed to the priest.

An overwhelming terrible omen was revealed to the priest.

What is the rule?

Answer

The article changes based on the word immediately following, not necessarily on the noun. There is no difference in meaning between "a" and "an" - the distinction is used to preserve an alternation between vowels and consonants when the sentence is spoken aloud.

Be aware that speakers of American and British English observe different rules (mainly because we can't agree on whether to pronounce the letter H or not!)

In your example:

A Hypertext Markup language. An Hypertext markup language.

an American speaker (and, to be honest, most British speakers) would find the first one correct, while an exaggerated aitch-dropper would use the second.

In these examples:

An omen was revealed to the priest.

A terrible omen was revealed to the priest.
An overwhelming terrible omen was revealed to the priest.

all three are correct as written, whether the speaker is American or British.

http://en.wikipedia.org/wiki/Article_(grammar)#Indefinite_article