We place ‘a’ and ‘an’ (known as indefinite articles) before words to indicate something or someone non-specific, as in ‘a study’ or ‘an invitation’ – we are not referring to any particular (specific) study or any particular invitation.
We use ‘a’ before a word that starts with a consonant, or hard sound, such as the ‘b’ in ‘a book’, the ‘h’ in ‘a haircut’, the ‘p’ in ‘a perfect result’ and the ‘v’ in ‘a very good proposal’.
We use ‘an’ before a word that starts with a vowel, or soft sound, such as the ‘a’ in ‘an ally’, the ‘e’ in ‘an effective strategy’, the ‘i’ in ‘an irritatingly boring speaker’, the ‘o’ in ‘an overdue account’ and the ‘u’ in ‘an upcoming conference’.
The letters ‘h’ and ‘y’ can be pronounced as either a consonant or a vowel, as in ‘Are you a happy camper?’ and ‘Is that a yes?’ – ‘h’ and ‘y’ pronounced as consonants – and ‘An honourable member answers honestly,’ and ‘Is that an Yves St Laurent creation?’ – the ‘h’ and ‘y’ are pronounced as vowels.