When do we use 'a' or 'an' before words starting with 'f'?

When do we use 'a' or 'an' before words starting with 'f'?

We use 'a' before words starting with 'f' when the 'f' sound is pronounced as a consonant. We use 'an' before words starting with 'f' when the 'f' sound is pronounced as a vowel.