class 5: indefinite article "a/an"

Is it "a" or "an"?

first letter of the noun is a vowel: an

an apple

first letter of the noun is a consonant:

a book

Be careful with the letter "u"

an aumbrella
BUT: a unit

http://static.twoday.net/englisch2010/files/lueckentext_a_an.htm
http://static.twoday.net/englisch2010/files/grammatikquiz_a_an.htm
http://www.englisch-hilfen.de/en/exercises/nouns_articles/article_a.htm
http://www.englishexercises.org/makeagame/viewgame.asp?id=1203