To say for instance that something is good or too expensive FOR us, we use  /duì/ 

 /duì/ is a preposition of a confrontation with some phenomenon

When you want to say that something is too expensive FOR you or that smoking isn’t good FOR health, you use 对 /duì/.

对 /duì/ if FOR – but not in the sense that we have bought something for someone, but in the situations when we are describing attitude TO something or influence of one thing TO another.

Basically we take a phenomenon and put it OPPOSITE us (or someone or something else), we CONFRONT ourselves with it and say what the result of that confrontation is.

 

EXAMPLES

4am is too early for me.
早上 四 点 对 我 太 早。
<Zǎoshang sì diǎn duì wǒ tài zǎo.>
When it is too fast, it is too difficult for me.
要是 太 快,对 我 太 难。
Yàoshi tài kuài, duì wǒ tài nán.
I am very disapointed in you.
我 对 你 很 失望。
Wǒ duì nǐ hěn shīwàng.
I think that this has no influence on me.
我 想 这个 对 我 没有 影响。
Wǒ xiǎng zhège duì wǒ méiyǒu yǐngxiǎng.
Chinese music is too slow for me.
中国 音乐 对 我 太 慢 了。
<Zhōngguó yīnyuè duì wǒ tài màn le.>
This isn't good for me.
这个 对 我 不 好。
<Zhège duì wǒ bù hǎo.>
It is good for me to work in a global company.
在 跨国公司 工作 对 我 很 好。
Zài kuàguó gōngsī gōngzuò duì wǒ hěn hǎo.
He is nice to me.
他 对 我 很 好。
<Tā duì wǒ hěn hǎo.>