One day Li Ming caught a little bird. He tied it with a piece of thread and played with it happily. At that time，he saw a little girl crying by the roadside. A policeman saw her，came over and asked her why she was crying and where her house is. When he learned that girl had got lost and she was then looking for her mother，he decided to lead the girl to be back home.
Seeing this Li Ming thought the little bird also had its mother and he should set the little bird free so that it could be with its mother again. Then Li Ming untied the thread and returned the little bird to the blue sky.
牛津实用英语语法:149 表示无义务的各种形式
牛津实用英语语法:144 must和have to的各种形式
牛津实用英语语法:151 must，have to和 need用于疑问句
牛津实用英语语法:139 ought的各种形式
牛津实用英语语法:150 need not与其他各种形式的区别
牛津实用英语语法:155 need意指require（需要）
牛津实用英语语法:136 can和 be able的各种形式
牛津实用英语语法:146 need not和 must not用于现在时和将来时
牛津实用英语语法:171 think， assume和expect
牛津实用英语语法:119 have+宾语+过去分词