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

Smoking isn't good for the body.
抽烟 对 身体 不 好。
<Chōuyān duì shēntǐ bù hǎo.>
For me thursday is the best.
对 我 来 说,星期四 最 方便。
<Duì wǒ lái shuō, xīngqīsì zuì fāngbiàn.>
He is nice to other people.
他 对 别人 很 好。
<Tā duì biérén hěn hǎo.>
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.
8pm is too late for me.
晚上 八 点 对 我 太 晚 了。
<Wǎnshàng bā diǎn duì wǒ tài wǎn 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.
4am is too early for me.
早上 四 点 对 我 太 早。
<Zǎoshang sì diǎn duì wǒ tài zǎo.>