a和an都是一个/只/件的意思.
an是用在所用响音开始的名词,那便是:A,E,I,O,U.
例:anapple;anelephant;anorange;anuglyduckling(丑天鹅);anunusualsituation(一个不寻常的情形).
不是响音开始的则用"a"如:aman;apen;acar等等.
用法:
Anelephantislargethananorange.一只大象比一只橙大.
Thatunusualsituationisnowundercontrolled.那不寻常的事态已受控制.
Shewasseenasanuglyducklingwhenshewaslittle.她小的时候被视为一只丑天鹅.
Anangleof90degreesisastraightangle.一个90度的角是个直角.(这个例很重要,例中的两个"angle"一个用"an"一个则用"a".你研究一下便会明白"a"和"an"的用法了)
至於"the"的用法如楼上:
Iusetohaveadog.Thedoglikedeatingandgrewsobig.
Iwalkedtoschooleverymorning.Theschoolisveryfarawayfrommyhome.
Ihaveatelevisioninmybedroom.Thetelevisionisquitesmall.