a 和 an 都是一個/只/件 的意思.
an 是用在所用響音開始的名詞,那便是 :A,E,I,O,U.
例:an apple；an elephant；an orange；an ugly duckling(丑天鵝)；an unusual situation (一個不尋常的情形).
不是響音開始的則用 "a" 如 :a man；a pen；a car 等等.
用法:
An elephant is large than an orange.一只大象比一只橙大.
That unusual situation is now under controlled.那不尋常的事態(tài)已受控制.
She was seen as an ugly duckling when she was little.她小的時候被視為一只丑天鵝.
An angle of 90 degrees is a straight angle.一個90度的角是個直角.(這個例很重要,例中的兩個"angle"一個用"an"一個則用"a".你研究一下便會明白"a"和"an"的用法了)
至於"the "的用法如樓上:
I use to have a dog.The dog liked eating and grew so big.
I walked to school every morning.The school is very far away from my home.
I have a television in my bed room.The television is quite small.