Write a or an:
- a window
- a horse
- an airport
- a new airport
- an organization
- a university
- an hour
- an economic problem
The use of "a" or "an" depends on the sound that follows the article. "A" is used before words that begin with a consonant sound, while "an" is used before words that begin with a vowel sound. In the case of "hour" and "economic", the pronunciation of the words begins with a vowel sound, so "an" is used before them.