怎样分辨三单和非三单

一般现在时主语单三人称时的动词变化

①在一般现在时中，当主语为第三人称单数时，动词要用“s”型（即第三人称单数形式）。

②所谓动词“s”型的构成，可按名词变复数的规则来记，即：

(1)在动词尾直接加 s。如：

play—plays， want—wants， work—works， know—knows， help—helps，get—gets

(2)以字母s、x、ch或o结尾的动词加-es；如：

guess—guesses， fix—fixes， teach—teaches， brush—brushes， go—goes，do—does，watch—watches，catch—catches

(3)以辅音字母＋y结尾的动词，先变y为i，再加-es。如：

study—studies，carry—carries，fly—flies，worry—worries

③④强调如何将主语是第三人称单数的肯定句变为否定和疑问句。现举例说明：

1）His mother works in a factory．

His mother doesn’t work in a factory．

Does his mother work in a factory？

2）Mr Li teaches us English．

Mr Li doesn’t teach us English．

Does Mr Li teach you English？

3）My brother studies maths well．

My brother doesn't study maths well．

Does your brother study maths well？