Most British English speakers say a herbaceous since the “h” is typically silent in British English. Americans, however, choose the article (a/an) that best suits their own pronunciation.

When speaking American English, we usually say an herbal because the initial letter of this word, the “h”, is silent. The British, however, commonly use the article "a" since the “h” is sounded in British English.

In American English, it’s an herb since the “h” is silent. In Britain, by contrast, the “h” is typically sounded, so they say “a herb”.

It’s an heirloom. The word heirloom begins with a silent "h", so we use the indefinite article "an". Remember, the rule for using "a" or "an" is based on pronunciation, not spelling.

It’s an heiress. The rule for using "a" or "an" is based on pronunciation. Since "heiress" is a word with a silent "h", it takes the article "an".

It’s an heir. Since the first sound of the word heir is a vowel sound (the letter “h” is silent), we use the indefinite article “an”.

It’s a uvula. The rule for using “a” or “an” is based on pronunciation—not spelling. If the following word has a consonant sound, then the indefinite article to use is “a”.

It’s a user; thus, we say “a user-friendly”, “a user interface”, “a user fee”, etc. The word “user” begins with a consonant sound, a “you” sound, and therefore takes the indefinite article “a”.

It’s a useful... The rule for using “a” or “an” is based on pronunciation, not spelling. Since the word useful starts with a consonant sound (a “you” sound), it takes the indefinite article “a”.