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.>
This is a bit complicated for the men from the south.
这个 对 南方 的 男人 有点 难。
Zhège duì nánfāng de nánrén yǒudiǎn nán.
For me thursday is the best.
对 我 来 说,星期四 最 方便。
<Duì wǒ lái shuō, xīngqīsì zuì fāngbiàn.>
I am very disapointed in you.
我 对 你 很 失望。
Wǒ duì nǐ hěn shīwàng.
Flying is bad for Earth.
飞行 对 地球 不 好。
Fēixíng duì Dìqiú bù hǎo.
It is good for me this way.
这样 对 我 很 好。
Zhèyàng duì wǒ hěn hǎo.
He is nice to other people.
他 对 别人 很 好。
<Tā duì biérén hěn hǎo.>
This isn't good for me.
这个 对 我 不 好。
<Zhège duì wǒ bù hǎo.>