Choose a/an by sound (an hour, a university), not spelling.
Use 'an' before a vowel SOUND; use 'a' before a consonant SOUND.
Do not choose by the first letter; choose by the first sound.
Is there a/an ...? / Do you have a/an ...?
💡 Examples: an apple (vowel sound), a book (consonant sound), an hour (silent h), a university (starts with /juː/), an MBA (starts with vowel sound /ɛm/).
✓an hour
✗a hour
The 'h' is silent, so the word begins with a vowel sound.
✓a university
✗an university
It begins with the consonant sound /j/ (you-niversity).
✓an MBA
✗a MBA
The letter name starts with a vowel sound (/ɛm/).