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

Flying is bad for Earth.
飞行 对 地球 不 好。
Fēixíng duì Dìqiú bù hǎo.
When it is too fast, it is too difficult for me.
要是 太 快,对 我 太 难。
Yàoshi tài kuài, duì wǒ tài nán.
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.
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.
This isn't good for me.
这个 对 我 不 好。
<Zhège duì wǒ bù hǎo.>
It is good for me this way.
这样 对 我 很 好。
Zhèyàng 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.>
He is nice to me.
他 对 我 很 好。
<Tā duì wǒ hěn hǎo.>