(误)She was playing piano.
(正)She was playing the piano.（演奏乐器，如piano，violin，乐器前面要加定冠词the。）
(误)The children go to the school every morning.
(正)The children go to school every morning.（go to school是习惯用语，不加the。）
(误)She has a beautiful hair.
(正)She has beautiful hair.（hair指头发的整体，是不可数名词，前面不加a。）
(误)She likes to watch the TV.
(正)She likes to watch TV.（看电视，习惯上TV前不加the。）
(误)She was listening to radio in the room.
(正)She was listening to the radio in the room.（听广播，习惯上radio前面加the。）
(误)Nile River is the longest river in Egypt.
(正)The Nile River is the longest river in Egypt.（在河流，山脉等名称前要用定冠词。）
(误)It's a nice weather.
(正)It is nice weather.（weather是不可数名词，前面不加a。）
(误)See you again the next week.
(正)See you again next week.（next week表示将来；the next week表示过去。）
(误)The history is a very interesting subject.
(正)History is a very interesting subject.（学科前一般不加冠词。）
(误)He came here by a train.
(正)He came here by train.（by后接交通工具，中间不加冠词。）
(误)You may see the full moon tonight.
(正)You may see a full moon tonight.（月亮，太阳等独一无二的物体，当有形容词修饰时可加a。）
(误)She went to the Paul Street just now.
(正)She went to Paul Street just now.（街道名称前一般不加the。）
(误)Even child can do it.
(正)Even a child can do it.（不定冠词＋单数名词表示某类人或物。）