We say “an old man” but “a young child”. What is the rule for using “a” and “an”?

The rule is that we use “a” before a consonant sound and “an” before a vowel sound. Note that it’s the sound that matters, not the letter the word begins with.

Examples:

Words that begin with “h” or “u” can sometimes be confusing, but you should stick to the above rule.

We say “an hour” because we pronounce “hour” without the h-sound. Similarly we say “an honourable person” because “honourable” is pronounced “onrable”.

In British English the word “herb” is pronounced with the h-sound – so we say “a herb”, but Americans tend to pronounce it as “erb”, so they write “an herb”.

One area of debate is how to use the word “historic”. Do we say “an historic achievement” or “a historic achievement”? On Google “an historic achievement” gets over 94,000 results, while “a historic achievement” gets just over 80,000 results – so, on that measure it seems “an historic…” wins – even though we usually pronounce the h-sound.

We use “a” with words like “university”, “utopia” and “European” because, although they start with a vowel, they are pronounced with a consonant sound – “you-niversity”, “you-topia”, “your-opean”.

But words like “uncle” and “onion” start with a vowel sound, so we say “an uncle”, “an onion”.

But the word “one” is pronounced “wun”. So it takes “a”. For example, “a one-track mind”.

Abbreviations and acronyms also take “a” or “an” depending on their pronunciation – so we say “a NATO member”, but “an MBA” – where NATO is pronounced “nayto” and MBA is pronounced “em-bee-ay”.