Use an before a silent 'h'. Use a before an aspirated 'h'.
- an heir
- an honour
- an hour
- a hotel
- a historian
With abbreviations, be guided by pronunciation. If the start of the abbreviation sounds like a vowel, use an. If it sounds like a consonant, use a.
- a BSc
- an EdD
- an LLB
- an MB ChB
- an MPH
- an MSc
- a PGCertGP
- a PhD