Ein - is an indefinite article which used for nouns of neutral and masculine gender Eine - for nouns of female gender Depending on case, gender and number you can find also other form of this article^ einen, einer, einem, eines

Das ist eine Katze. ”Katze“ is a feminine noun (Die Katze) , in this sentence "Katze" is subject, Nominativ, so the indefinite article "ein" has a feminine End with "e". Ist es ein Apfel?, "Apfel" is a Masculine noun (Der Apfel), in this sentence "Apfel" is subject, Nominativ, so the indefinite article "ein" stays "ein". all Adjective, Article and pronoun need to changes along with the Noun (depends on if it's Nominativ, Akkusativ, Dativ, Genitiv and the Noun's gender, and Singular or Plural ) for example: Der Rock (Masculine noun): Nominativ: der rote Rock (definite article ), ein roter Rock (indefinite article), roter Rock(No articles) Akkusativ: den roten Rock (definite article ), einen roten Rock (indefinite article), roten Rock(No articles) Dativ : dem roten Rock (definite article ), einem roten Rock (indefinite article), rotem Rock(No articles) Genitiv : des roten Rocks (definite article ), eines roten Rocks (indefinite article), roten Rocks(No articles) Die Bluse ( feminine noun): Nominativ: die gelbe Bluse(definite article ), eine gelbe Bluse(indefinite article), gelbe Bluse(No articles) Akkusativ: die gelbe Bluse(definite article ), eine gelbe Bluse(indefinite article), gelbe Bluse(No articles) Dativ : der gelber Bluse(definite article ), einer gelben Bluse(indefinite article), gelber Bluse(No articles) Genitiv : der gelben Bluse(definite article ), einer gelben Bluse(indefinite article), gelber Bluse(No articles) and so, i hope i did not confuse you...here is a website for german grammatik. have fun. http://www.grammatikdeutsch.de/html/eigenschaftswort-adjektiv.html