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 other people.
他 对 别人 很 好。
<Tā duì biérén hěn hǎo.>
His attitude to me has changed.
他 对 我 的 态度 变 了。
Tā duì wǒ de tàidu biàn le.
This movie had a big influence on me.
这 部 电影 对 我 很 有 影响。
Zhè bù diànyǐng duì wǒ hěn yǒu yǐngxiǎng.
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.
He is nice to people.
他 对 人 很 好。
<Tā duì rén 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.>
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.