Although we all heard about this one in school, the correct use of a and an can cause a lot of confusion. There is a very clear rule when to use a and when to use an. However, this rule seems to be often forgotten or remembered incorrectly.
a is used when the following word starts with a consonant sound.
Examples: a word, a sound, a mouse.
an is used when the following word begins with a vowel sound.
Examples: an end, an apple, an orange.
The important point here is that it is the sound, not the spelling, that decides between a and an. Hence, the following expressions are correct:
a university, a European, a useful hint, a one-time thing, an honest mistake.
My suspicion is that many advanced speakers incorrectly remember that the rule is about spelling rather than pronunciation, and hence use an for any word that starts with a vowel, and a for the rest. However, if you read the above examples out loud, you will quickly notice that the rule based on pronunciation makes much more sense.
Even if you remember the above rule, things get a bit tricky when you are not a native speaker, because the choice between a and an depends on pronunciation. In the above examples, we had the phrase “an honest mistake”. Similarly, you say “an heir” rather than “a heir”, because the letter h in these words is silent. However, there are other words starting with an h which require the article a because the h is not silent. For example, you say
a human, a humorous story, a humid climate
Words that start with an h are very tricky for French, Spanish and Italian native speakers, because in those languages the h is silent or even completely dropped in words that also exist in English. As an example, consider the word humid. It is pronounced as [ˈhjuːmɪd], and therefore we have to write “a humid climate” rather than “an humid climate”. However, the French word humide, and Spanish humedo, and the Italian umido either havea silent h or no h at all. Consequently, native speakers of those languages often find it hard to correctly use a and an in English.
Let me finish this post with two interesting issues:
The word herb is pronounced [hɜːb] in British English, but often as [ɜːrb] in American English. Hence, you would use “a herb” in British English, but “an herb” in American English.
If you use abbreviations, it is again the pronunciation of the abbreviation rather than the spelling, or the pronunciation of the word you are abbreviating, that decides between a and an.
an FM radio (but a frequency modulation radio),
an HTML document (but a hypertext …),
an LOL (but a laugh-out-loud),
an ATM card