''An'' is used when the following word starts with a vowel. For example: An airplane. ''A'' is used when the following word starts with a consonant. For example: A window.

Daniela, here is some more help concerning your question about "a" and "an". Paul's addition about using "an" when the following word starts with a vowel sound is correct. To clarify further, one would also need to use "a" when the following word starts with a consonant sound. The real rule is this: You use the article “a” before words that start with a consonant sound and “an” before words that start with a vowel sound. For example, "He has a unique point of view on the subject and talked about it for an hour". The “u” in “unique” makes the “Y” sound—a consonant sound—therefore you use “a” as your article, while the “h” in “hour” sounds like it starts with “ow”—a vowel sound, therefore you use "an" as your article. The rule as stated by Liam can be of benefit to you as a starting point until your are able to learn about the various, somewhat unique "exceptions to the rule" type words: those that either start with a vowel letter but have a consonant sound at the beginning, or those that start with a consonant letter but have a vowel sound at the beginning. English has two types of articles: 'the" and "a/an", "the" = definite article, while "a/an" = indefinite articles. Here are some links about this subject: https://owl.english.purdue.edu/owl/resource/540/01/ https://owl.english.purdue.edu/owl/resource/591/01/ http://www.edufind.com/english-grammar/indefinite-articles/ http://www.usingenglish.com/glossary/indefinite-article.html That last link has some good beginner, intermediate, and advanced quizzes. Here are some examples of using a/an with the exception type words: a european, a university, a unit, a union (y sounds); an hour, an honor (silent "h"); a one time occurrence (one starts with a "w" sound).