《三单式》

请问一下，为什么三单式后加es或s？

解题思路： 如下
解题过程：
同学您好，动词的第三人称单数是一般现在时的范畴，老师总结了一下一般现在时的用法，这对您掌握动词的第三人称单数是有帮助的。
一般现在时
①一般现在时表示反复、经常发生的动作或习惯用。常与always, usually, often, sometimes, every day, every year, every week, once a week（一周一次）等连用。
I go to school by bike every day. 我每天骑自行车上学。
Lily goes to school by bike every day. 丽丽每天骑自行车上学。
②客观真理，客观存在，科学事实。
　The earth moves around the sun.
Shanghai lies（位于）in the east of China.
③现在时刻的状态、能力、性格、个性。
　I don’t want so much.
Ann Wang writes good English but does not speak well.
I can swim.
构成：
◎主语是I , we, you(你), you(你们), they 及两个或两个以上的人或物作主语：
肯定句：主语+动词原形+其他
I play basketball after school.
They play basketball after school.
Tom and his friends play basketball after school.
否定句：主语+don’t +动词原形+其他
I don’t play basketball after school.
They don’t play basketball after school.
Tom and his friends don’t play basketball after school.
一般疑问句：Do + 主语+动词原形+其他?
肯定回答：Yes, 主语+ do.
否定回答：No, 主语+ don’t.
Do you play basketball after school? Yes, I do. /No, I don’t.
Do they play basketball after school? Yes, they do. /No, they don’t.
Do Tom and his friends play basketball after school? Yes, they do. /No, they don’t.
◎主语是he, she, it 及单个人或物作主语：
肯定句：主语+动词第三人称单数+其他
He plays basketball after school.
Tom plays basketball after school.
否定句：主语+doesn’t +动词原形+其他
He doesn’t play basketball after school.
Tom doesn’t play basketball after school.
一般疑问句：Does + 主语+动词原形+其他?
肯定回答：Yes, 主语+ does.
否定回答：No, 主语+ doesn’t.
Does he play basketball after school? Yes, he does. / No, he doesn’t.
Does Tom play basketball after school? Yes, he does. / No, he doesn’t.
动词原形变第三人称单数的规则与名词单数变复数大致相同。
1、 大多数动词在词尾加“s”。如：
stop－stops; make－makes；read－reads; play－plays
2、以辅音字母加“y”结尾的，要先将“y”变为“i”，然后在加“es”。 如：
fly－flies； carry－carries； study－studies ; worry－worries
3、以“s, x, ch, sh”结尾的，在词尾加“es”。如：
teach－teaches ; watch－watches
4、以“o”结尾的动词，加“es”。 如：
go－goes; do－does
5. have 的第三人称单数是has。
以上这些变化是规定，记住变化规则就OK了。
最终答案：略