Why not do sth.
Why don't you do STH.
Would you please do sth. / Would you please not do sth. 每当我需要你的帮助时,你总是拒绝了。
Why not do it this way? It'll save you a lot of trouble. 干吗不这样做?可以省你很多麻烦。
Now that you have decided to do it, why not do your best? 既然你已经决定做它了,为什么不尽力?
If I'm gonna built a time machine into a car why not do it somestyle. 既然要把车改造成时光机器,。