use of an

[daveh]

when you use an are there any exceptions to the rule ie an is used before a vowel sound but what about before sms / std

[Casiopea]

Welcome, David.

The rule doesn't apply to a word's written form. It applies to a word's spoken form. "sms" and "std" are written with a consonant-initial, yes, but that consonant, "s", is pronounced with a vowel. Say the alphabet, a b c d . . . s t u . . .

Note, United States is written with a vowel, "U", but that vowel is pronounced [ju:], with consonant-initial [j] <y>. So, a United States member, not an United States member.

Hope that helps.

[rewboss]

It simply depends on how you pronounce the word. Look at this:

An NBC journalist asked a NATO spokesman for his views.

"An NBC journalist" because we pronounce "NBC" as letters -- "en, bee, see". But "A NATO spokesman" because we pronounce "NATO" as if it were a word: "nay-toe".