1
be mean to sb
mean 作形容词可表示“刻薄的”，主语通常为人，be mean to sb 表示“对某人刻薄/不友善”。
Don't be mean to your little sister.
别对你妹妹那么刻薄。
不友善的；刻薄的；恶意的
先理解核心含义，再看它在真实句子里的用法。
不友善的；刻薄的；恶意的
Unkind or intending to harm; pettily offensive.
Don't be mean to your little sister.
别对你妹妹那么刻薄。
质量或程度低；劣质的；差的；破旧的。
Low in quality or degree; inferior; poor; shabby.
具有平均值；平均的。
Having the mean as its value; average.
缺乏尊严；无荣誉；卑鄙；下贱。
Without dignity of mind; destitute of honour; low-minded; spiritless; base.
价值低微；不值得重视；可鄙；可耻。
Of little value or worth; worthy of little or no regard; contemptible; despicable.
He is so mean that he never shares his snacks.
他非常吝啬，从不分享他的零食。
My boss is very mean with money; he won't give us a raise.
我的老板对钱很吝啬，他不会给我们加薪。
It was mean of her to laugh at his mistake.
她嘲笑他的错误，真是太刻薄了。
mean 作形容词可表示“刻薄的”，主语通常为人，be mean to sb 表示“对某人刻薄/不友善”。
Don't be mean to your little sister.
别对你妹妹那么刻薄。
这是一个以 it 作形式主语的句型，真正的主语是后面的 to do sth，of 后接做出该行为的人，用来评判该行为刻薄。
It was mean of her to laugh at him.
她嘲笑他，真是太刻薄了。
Not this
He is more mean than I expected.
Use this
He is meaner than I expected.
mean 是单音节形容词，比较级应直接加 -er 变成 meaner，而不是用 more mean；more 只用于多音节形容词或部分双音节词。
Not this
She was always mean with me at school.
Use this
She was always mean to me at school.
表示“对某人刻薄”时，mean 后接介词 to（mean to somebody），不能用 with；with 虽可搭配 mean 表示“在花钱上吝啬”，如 be mean with money，但意思完全不同。