Spelling guidelines
2. A or an?
Although the basic rule for when to use a and when to use an is straightforward, many advanced users of English become confused when having to decide between a and an before a word that starts with h, and in some cases also e, o or u. For the vowels the rule is that if the word is pronounced as if the first letter were a consonant, specifically w or y, then you should write a; otherwise you should write an. This rule has no exceptions:
| a ewe | an early-warning system | |
| a one-off arrangement | an only child | |
| a united front | an understandable mistake |
Actually, the rule for h is equally straightforward: if the h is silent, you use an; otherwise you use a:
| an hour | a horrifying thought | |
| an honest man | a hopeless case | |
| an heiress | a happy thought |
However, some learners become confused when they see texts which include expressions like an historic occasion, an habitual grin. This is due to an old-fashioned convention which holds that an is used before a non-silent h if the first syllable of the word is unstressed. But since the convention has now almost fallen into disuse, we can say that, like the rule for the vowels, the basic rule for h has no exceptions.
A second problem concerns abbreviations that begin with a consonant but which are pronounced as if they begin with a vowel, as in MA (Master of Arts). There might appear to be a choice here. An argument in favour of a MA is that M is a consonant and the article must therefore be a. An argument in favour of an MA is that the pronunciation decides the spelling: because you say an MA, you write an MA. This second form is definitely the more popular one.
A third and final point relates to mistakes like these:
 | a interactive frame |
| an too insistent member |
For advanced users these are serious mistakes, of course, in that the most basic of rules has been broken. Yet they do occur. What presumably happens here is that you start with, say, a highly interactive frame and an insistent member, and then decide to remove highly and add too, respectively. This process has become such a mechanical one that you may well forget that the form of any indefinite article in the immediate vicinity has to be adapted accordingly.