This is always one of those situations you just don’t think about when you’re speaking, but when you’re writing, you wonder, ‘It looks right, but is it? That’s not how you say it!’
We all know the general rule is to apply ‘an’ before a vowel (a, e, i, o, u) and ‘a’ before a consonant. You think that would be the rule in writing, too, and it is, until you write a sentence like:
‘It takes about an hour to get to Liverpool from Stoke-on-Trent.’
Then you scratch your head and say to yourself, hang on, the letter ‘H’ in English is a consonant. You might say ‘an hour’ but surely ‘a hour’ is right?
Unfortunately, for consonants like ‘H’, that’s where the rule is chucked out of the window with the contempt the English language can sometimes dish up.
Read this sentence aloud:
‘It takes about a hour to get to Liverpool from Stoke-on-Trent.’
That might look right technically, but it sounds daft, doesn’t it? In this context, the ‘H’ is silent, even though the general rule is to use ‘a’ as in: ‘a house’*; ‘a horse’*; ‘a hospital’*. We can hear the ‘H’ when we say those words, so we know they’re consonants. We say ‘hour’ like it doesn’t begin with ‘H’, like it begins with its second letter ‘O’, and so we have to treat it like it’s a vowel.
And then there’s the letter ‘M’!
‘An MRI scan.’
Not ‘A MRI scan’? Why not? Because when we get to ‘M’ as we chant the alphabet, we actually say ‘EM’ like it begins with the vowel ‘E’, therefore ‘an’ in this context would be correct. In fact, using ‘an’ would be almost always correct where it would precede an abbreviated word in speech starting with a consonant: ‘They sent an ROV into the sea’.
Vowel word examples where the general rule is broken usually begin with ‘U’ and ‘O’:
A uniform (where ‘U’ sounds like ‘Y’ as in ‘yew’)
A one-way ticket (where ‘O’ sounds like ‘W’ as in ‘window’)
So it’s all to do with sound: if a word sounds like it starts with a vowel when you say it, then treat it like a vowel and use ‘an’; if a word sounds like it starts with a consonant, then use ‘a’.
It’s that simple, really.
*Unless like me you’re a Scouser, when you’d actually say, ‘an ‘orse’, ‘an ‘ouse’ or ‘an ‘ospital’!