be good at意思比较简单，就是“善于，擅长，在......方面做得好”之意，at表示“方面”。例如：

I'm good at history but poor at arithmetic.  我的历史不错，但算术不太好。

The boy is good at doing a sum in his head.  这男孩善于心算。

The writer is not very good at writing dialogues.  这个作家不太擅长写对白。

I am not good at organic chemistry.  我的有机化学学得不好。

She is no less good at swimming than Mary.  她和玛丽一样擅长游泳。

He is good at teaching his students to reason.  他擅于教导学生如何推理。

但是，be good to和be good for就不那么简单了，请看我以前写的这个解答，也许对你有帮助。

good for 还是 good to?

学生问：有一次见到一本教辅书上讲，跟useful, important, necessary这些词一样，good之后用介词to或for没有区别，可以互换。请问老师，这样说对吗？

这种说法是不对的。因为good有不同的含义，所使用的介词也不尽相同，而且to和for的意思也有差别。good在句中用作表语时，接for表示“有益的；有效的；合适的；胜任的”，good for还有“付得起，价值是”之意。接to表示“对......友善的，友好可亲的”。

下面，我们先看一个英语幽默故事：

A man was troubled by mice. They ran about all over his house at night. In the daytime, they just stayed under the floor.

The man thought, “What I need is a cat, a good cat that can catch mice.” He knew of a man who had cats for sale. So he went to see him. “Do you have a cat that is good for the mice?” he asked the man. “Well, yes. I do happen to have a cat that is good to the mice,” replied the man. “I’ll take it,” said the first man.

He paid for the cat and took it home. A week later, the cat didn’t catch any mice, but it played with them like old friends. So the man went back again with the cat under his arm. They had a quarrel because they both took the meaning of “good” as they needed.

上面的故事十分有趣。买猫人是想买一个good for mice的猫，然而，那个卖猫人有一个good to mice的猫，最后结果必然是一场quarrel。

下面，我们分析一下故事中的两个关键句子：

“Do you have a cat that is good for the mice?”

“你这儿有没有善于捉鼠的猫？”

“I do happen to have a cat that is good to the mice.”

“碰巧我还真有一只对老鼠不错的猫。”

通过文字游戏，在买猫人和卖猫人之间，发生了意想不到的效果。

【重要提示】

1. good表示“适合，有效，胜任”意思时，用be good for；

2. good表示“仁慈，善良”意思时，用be good to；

3. good表示“对......有益处”时，用be good for 和be good to 都可以。但前者用得相对多一些。

试比较以下例句：

1. 老太太对那些男孩子和蔼可亲。

The old lady is good to the boys.（正确）

The old lady is good for the boys.（错误）

2. 这药对该疾病有效果。

The medicine is good to the disease.（错误）

The medicine is good for the disease.（正确）

3. 这条金链刚好与吉姆的手表相匹配。

The gold chain was just good to Jim's watch.（错误）

The gold chain was just good for Jim's watch.（正确）

4. 约翰不适合这项工作。

John is not good to the job.（错误）

John is not good for the job.（正确）

5. 体育运动有益健康。

Athletic sports are good to your health.  （正确）

Athletic sports are good for your health. （正确）

下面，列举一些含有good for和good to的句子，以示比较。

Fresh air and exercise are good for the health. 新鲜空气和运动有益于健康。

Whisky can be good for you if taken in moderation.  威士忌酒如饮用适度是有益的。

It's not a good film, but it's good for a laugh. 这部电影并不好,但能引人发笑。

Smoking is not good for your health. 吸烟对你的健康没有好处。

This book is good for beginners. 这本书对初学者很有用。

A bowl of hot chicken soup will be good for you. 喝一碗热鸡汤对你有好处。

Could job-hopping be good for your career? 跳槽对你的职业生涯有益吗?

Doing exercises in the open air is good for your health.  露天锻炼身体有益你的健康。

Whisky is good for you if take in moderation. 威士忌酒如饮用适度是有益的。

The weather looks good for our sports meet. 天气适宜我们举行运动会。

This medicine is good for your cough. 这种药对你的咳嗽很有效。

This soup is very good for stomach pain. 这汤对胃疼很有帮助。

He is good for the rent, I am sure. 我相信她付得起租金。

He is always regarded as good for nothing. 他被认为是个一事无成的人。

He bought a second hand bike good for 80 yuan. 他买了一辆值80元的二手自行车。

Everything is good for something. 天生一物必有用。

The teacher is good to all his students. 那位老师对他的学生都很好。

You have been so good to me in the past. 您过去一直对我非常好。

All the parents are good to their children. 天下父母都对自己的子女很好。

The young should be good to the old. 年轻人应该孝顺老人。

By and large, the company's been pretty good to me.  总的来说，公司对我一直很好。

The boss has always been very good to her. 老板对她一直非常好。

Julie is very good to me. We often talk about you. 朱利待我非常好，我们经常在一起谈到您。

My granny has always been good to me.  我外婆总是对我很好。

He was very good to me when I was ill.  我生病时他帮了我的大忙。

Keeping broadminded is good to your health. 保持心胸开阔有利于身体健康。

We should be good to old people. 我们应该善待老人。

She was grateful to him for being so good to her. 她对他的友好相对十分感激。

She was very good to the children. 她对孩子们非常慈爱。

Be good to me, for my hair went out of love for you. 你要对我好啊。卖掉头发是因为爱你呀。

We should be good to wild animals. 我们应该善待野生动物。

以上例句中，表示“对......有益处”的句子，for 跟to 可以互换。