An indefinite article indicates that its noun is not a particular one (or ones) identifiable to the listener. It may be something that the speaker is mentioning for the first time, or its precise identity may be irrelevant or hypothetical, or the speaker may be making a general statement about any such thing. English uses a/an, from the Old English forms of the number 'one', as its indefinite article. The form an is used before words that begin with a vowel sound (even if spelled with an initial consonant, as in an hour), and a before words that begin with a consonant sound (even if spelled with a vowel, as in a European). Example: She had a house so large that an elephant would get lost without a map.
For practice activities and lesson ideas on indefinite articles, look no further. Here at Busy Teacher, there are a variety of worksheets available that cover this topic; most of these are designed for beginner classes because this is usually covered early on in ESL courses. There are currently 93 indefinite article worksheets available in this section. You can use them as practice exercises for your students or as inspiration for your own worksheets. All worksheets are free and easy to download. This is a simple fill-in-the-blank practice activity for complete beginners. You can choose whether to use the black and white or color version. For another example look at this page which has a great activity for older students. Students listen to a song by Lily Allen and complete the worksheet. Honestly students will probably have to rely more on their understanding of articles than on their listening skills because the song is very fast paced. If there is time, you could even show students the related video. These two worksheets might not be your cup of tea but there are even more options that you might want to consider.