
a and an are indefinite articles, used before singular countable nouns to mean "one (non-specific)".
- a is used before consonant sounds: a book, a car, a university (pronounced /j/ sound).
- an is used before vowel sounds: an apple, an hour (h is silent).
我I 有have 一个a 问题question.
I have a question.
她She 是is 一名an 工程师engineer.
She is an engineer.
Note: Look at the pronunciation, not the letter: an hour (h is silent → an), a university (pronounced /j/ → a).