idea : 最普通常用词，几乎适用于任何方面的思维活动。
I've had an idea - why don't we go to the coast?
我有个主意——我们为什么不去海滨呢？
"Let's go swimming." "That's a good idea!"
“我们去游泳吧。”“好主意！”
If you have any ideas for what I could buy Jack, let me know.
我给杰克买什么东西好，如果你有什么主意请告诉我。
That's when I first had the idea of starting (= planned to start) my own business.
那是我首次有了自己创业的想法。
I like the idea of living in the countryside but I'm not sure I'd like the reality.
我觉得住在乡村的想法很好，但我不肯定是否喜欢这样一种现实。
She's full of bright (= good) ideas.
她满脑子好点子。
It was Kate's idea to hire bikes.
租自行车是凯特的主意。
It's not a good idea to drive for hours without a rest.
连续开车几个小时都不休息不是个好主意。
notion : 指的是一种模糊的，变化的莫测的想法，无可靠的基础，未经深思熟虑的观点。
The show's director rejects the notion that seeing violence on television has a harmful effect on children.
节目制片人们并不认为看到电视上的暴力场景会对孩子们造成不良影响。
I have only a vague notion of what she does for a living.
我不太清楚她靠什么谋生。