One and two is three.为什么用is而不是are?

One and two is three.结果是1个,用单数。
You and me are fine.结果是2个，用复数。
就这么简单

One and two表示1加2为一个整体，其结果是3，所以用单数
You and me 表示你和我两个人，所以用复数

One and two is three.=
One and two equals a number which is three

第一句的and是加的意思，2个主语是数字，所以不是复数，用is
第二局and是和的意思，你和我2个人，所以are

One and two 只有数没有量
You and me 可数