什么叫实义动词?为什么实义动词前加do的形式?

实义动词,也叫行为动词.就我们现在所学的内容来讲,实义动词所在的句子中一般不存在be动词.也就是在含有实义动词的句子变否定句或疑问句时,一定不能用be动词.那么,实义动词究竟怎样用呢?正如be动词随着主语的变化可变为am、is、are一样,实义动词在肯定句中,也要随着主语的变化而变化.当主语为第三人称单数he,she, it时,实义动词要变为相应的第三称单数；当主语为非第三人称单数I, you(你), we,you(你们) they时,实义动词要用动词的原形.而在变否定句和疑问句时,不能直接在动词上变,而是要借助另一类动词：助动词do、does.
（1）肯定句：
① I have a blue book.
② He has a brother.
③ She wants to be a teacher.
④ They like to play basketball.
(2)否定句：主语+don’t/doesn’t+动词原形.其中do/does为助动词,是来帮助实义动词构成否定或疑问句的,但加了do/does后,其后面的动词必须用动词原形.
① I have a blue book.（变为否定句）→I don’t have a blue book.
② He has a brother. （变为否定句）→He doesn’t have
③ She wants to be a teacher. （变为否定句）→She doesn’t want to be a teacher.
④ They like to play basketball. （变为否定句）→They don’t like to play basketball.
(3)一般疑问句：Do/Does +主语+动词原形+其他?句中加了do/does后,其后面的动词还是必须用动词原形.
③ My brother does his homework before supper. (变为一般疑问句)
① Your sister likes English best because it’s interesting. (对划线部分提问)
② He does his homework before supper. (对划线部分提问)
注意：当含有and引导的短语在句中做谓语或宾语,我们把这类句子变否定句时,必须把and变为or.如：