其实很简单，但首先要从动词开始说起。只有“实意动词”才会涉及到您说的问题，也就是“主谓一致原则”。句子分为“肯定句”、“否定句”和“疑问句”三种类型。在“肯定句”中，谓语动词要与主语保持一致：当主语为“三单（第三人称单数的简称）”时，谓语动词要采用“三单”形式，要在词尾加-s或-es，具体变法在此略去。举几个例子吧：
1) Li Weikang goes to school by bike every day.
2) Lucy has lunch at school.
3) Tom wears yellow T-shit and blue shorts.
4) He studies hard at school.
5) She comes to see me once a week.
当句子为“否定句”或“疑问句”时，需要加“助动词”来构成“否定”和“疑问”，但此时助动词后面的动词要用“原形”。还以上面的几个句子为例，分别变成“否定句”和“疑问句”，并作回答。
1) Li Weikang doesn't go to school by bike every day. （否定句）
Does Li Weikang go to school by bike every day? (疑问句）
Yes, he does. / No, he doesn't. （回答）
2) Lucy doesn't have lunch at school. （否定句）
Does Lucy have lunch at school? （疑问句）
Yes, she does. / No, she doesn't. （回答）
3) Tom doesn't wear yellow T-shirt and blue shorts. （否定句）
Does Tom wear yellow T-shirt and blue shorts? （疑问句）
Yes, he does. / No, he doesn't. （回答）
4) He doesn't study hard at school. （否定句）
Does he study hard at school? （疑问句）
Yes, he does. / No, he doesn't. （回答）
5) She doesn't come to see me once a week. （否定句）
Does she come to see me once a week? （疑问句）
Yes, she does. / No, she doesn't. （回答）
当然了，当主语为“非三单”时，若变“疑问句”和“否定句”也要加“助动词”的，举一个例子：“他们放学后在操场上踢足球”：
They play soccer on the playground after school. （肯定句）
They don't play soccer on the playground after school. （否定句）
Do they play soccer on the playground after school? （疑问句）
Yes, they do. / No, they don't. （回答）
现在你看明白了吗？我为了方便学生记忆，给他们编了两个“口诀”，做题时就不会出错了：
情态动词、助动词后，动词还原。（这指的是主语为“三单”时的用法）
用什么问，用什么答。（这指问句用的助动词与回答时所用的助动词，在“形”和“数”上保持一致，还有情态动词、there be等也都是如此。）举例：
1) There is a big tree in front of my house. （肯定句）
There isn't a big tree in front of my house. （否定句）
Is there a big tree in front of your house? （疑问句）
Yes, there is. / No, there isn't. （回答）
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