a / an: first mention, one of many

A / an means "one, and it does not matter which" or "one that you do not know yet". It introduces a new person or thing into the conversation. Your own language may leave this idea unmarked, so the article is easy to leave out.

a / an + (adjective) + singular countable noun

• I heard an interesting theory about first-born children.
• A colleague of mine comes from a small village.
• Is there a reason for this assumption?

a or an: listen to the sound

The choice depends on pronunciation. Words that begin with the sound /j/ (university, European, unique) or /w/ (one) take a, although they are written with a vowel. A silent h (hour, honest) and letters or numbers pronounced with a vowel sound (MBA, FBI, 8, 11, 80) take an.

• a + /j/ sound → a university, a European
• a + /w/ sound → a one-sided view
• an + silent h → an hour, an honest man
• an + letter name → an MBA, an FBI agent
• an + number → an 8-hour flight

• He works for a European charity.
• We waited for an hour and a half.
• She gave an 11-minute talk on gender bias.