英语中,三单形式到底是指什么啊?是全部变复数还是怎么样,

英语中,三单形式到底是指什么啊?是全部变复数还是怎么样,

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）