determiner
Used before one person or thing when you mean one, but not a particular one, and the next word begins with a vowel sound.
I need an umbrella.
She is an excellent doctor.
Good to know
Use an before a vowel sound, not simply before a vowel letter: an hour, but a university.
Common phrases & grammar
an idea · an example · an answer · an hour
an + singular noun
an + adjective + singular noun