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

His attitude to me has changed.
他 对 我 的 态度 变 了。
Tā duì wǒ de tàidu biàn le.
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 other people.
他 对 别人 很 好。
<Tā duì biérén hěn hǎo.>
Flying is bad for Earth.
飞行 对 地球 不 好。
Fēixíng duì Dìqiú bù hǎo.
8pm is too late for me.
晚上 八 点 对 我 太 晚 了。
<Wǎnshàng bā diǎn duì wǒ tài wǎn le.>
I am very disapointed in you.
我 对 你 很 失望。
Wǒ duì nǐ hěn shīwàng.
He is nice to me.
他 对 我 很 好。
<Tā duì wǒ hěn hǎo.>
He is nice to me.
他 对 我 很 好。
<Tā duì wǒ hěn hǎo.>