要 vs 想
yào vs xiǎng
Comparing "yao" and "xiang"
要 is a decision; 想 is a wish. And 想 plus a noun means "to miss".
要 = decided · 想 = would like
How it works
我要一杯水 orders the water. 我想喝水 says you fancy some. In a shop or restaurant, 要 is what you use.
With a noun, the two diverge completely: 我想你 means I miss you, not I want you.
The negatives split too: 不想 means you'd rather not, 不要 means don't.
Examples
wǒ yào yì bēi shuǐ
我要一杯水。
I'll have a glass of water.
wǒ xiǎng nǐ
我想你。
I miss you.
wǒ bù xiǎng qù, dàn shì wǒ yào qù
我不想去，但是我要去。
I don't want to go, but I have to.