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

I am very disapointed in you.
我 对 你 很 失望。
Wǒ duì nǐ hěn shīwàng.
He is nice to people.
他 对 人 很 好。
<Tā duì rén hěn hǎo.>
He is nice to me.
他 对 我 很 好。
<Tā duì wǒ hěn hǎo.>
4am is too early for me.
早上 四 点 对 我 太 早。
<Zǎoshang sì diǎn duì wǒ tài zǎo.>
His attitude to me has changed.
他 对 我 的 态度 变 了。
Tā duì wǒ de tàidu biàn le.
He is nice to me.
他 对 我 很 好。
<Tā duì wǒ hěn hǎo.>
Chinese music is too slow for me.
中国 音乐 对 我 太 慢 了。
<Zhōngguó yīnyuè duì wǒ tài màn le.>
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.