冠词 a / an 的用法
a用于辅音音素前 a useful book, a university, a “u”
Once a week have a swim/walk/talk/look/dance/drink/ rest have a cold/ headache / fever /cough have a good time have a try in a hurry after a while keep a diary go for a walk in a minute in a word in a short while
an则用于元音音素前 an hour, an honest boy , an “A E F H I L M N O R S X”
keep an eye on
1）特指双方都明白的人或物： Give me the book.
2）上文提到过的人或事： —Do you know the lady in blue? –Yes, she is a teacher of a university.
3）指世上独一物二的事物the sun ( sky / moon/ earth/ world/ nature/ universe )
4）单数名词连用表示一类事物，如：the dollar 美元; The lion is a wild animal.或与形容词或分词连用，
表示一类人：the rich /poor/ blind/ aged / living / impossible