the+单数名词为什么即可以表示特指又可以表示泛指

的确如此.我们来看两个例子.
1. 特指：
John saw a little boy sitting under a tree on his way home. He noticed that there was something moving in the tree. It was a snake! With little hesitation did he run to the boy. He grasped the boy and carried him away from the tree, and from the snake as well.
（在上面这一段中出现了三个加不定冠词的可数名词：a boy, a tree 和 a snake.当再次提到这三个名词时,不定冠词 a 都变成了定冠词 the.这就是特指,即前面提到过的可数名词再次被提及时就要用定冠词 the.）
2. 泛指：
Reading referred to "reading the prints", e.g. the book, only a couple of decades ago. Now, you read the book, the computer screen, and even the cellphone! Who knows what else you will be reading in the next few years, possibly something in the air right in front of you!
（在这一段中,定冠词 the 的使用很明显不是特指哪一本 book,或者哪一个 computer screen、哪一部 cellphone,而是指这几“类”可以读的可数名词.这就是所谓的“泛指”,即指某一“类”的可数名词.当然,你也可以不用 the+可数单数,而用复数.但用 the+可数单数 行文更为简洁明了.）
希望能对你有所帮助.