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

He is nice to me.
他 对 我 很 好。
<Tā duì wǒ hěn hǎo.>
8pm is too late for me.
晚上 八 点 对 我 太 晚 了。
<Wǎnshàng bā diǎn duì wǒ tài wǎn le.>
4am is too early for me.
早上 四 点 对 我 太 早。
<Zǎoshang sì diǎn duì wǒ tài zǎo.>
He is nice to me.
他 对 我 很 好。
<Tā duì wǒ hěn hǎo.>
This isn't good for me.
这个 对 我 不 好。
<Zhège duì wǒ bù hǎo.>
He is nice to other people.
他 对 别人 很 好。
<Tā duì biérén hěn hǎo.>
I am very disapointed in you.
我 对 你 很 失望。
Wǒ duì nǐ hěn shīwàng.
I am very satisfied with my new car.
我 对 我的 新的 汽车 很 满意。
Wǒ duì wǒde xīn de qìchē hěn mǎnyì.