✔ 最佳答案
The choice of using the articles "a" or "an" depends on the sound (as pronounced) of the countable noun that follows, but not the first letter of the noun.
Rules:
(1) Use "an" as the correct article, when the following noun pronounces a vowel sound on the first syllable;
(2) Use "a" otherwise.
Example:
1. X-ray: the first syllable sounds as [eks] which is a vowel sound, so you use article "an".
2. "s": it sounds as [es] which is also a vowel sound, so you use article "a".
3. used car: the first syllable sounds as [ju:] which is not a vowel sound, so you use the article "a" instead.
2011-10-21 11:08:53 補充:
Sorry, it was my typing mistake:
2. "s": it sounds as [es] which is also a vowel sound, so you use article "an".