people make a promise will do whatever they can (do).为什么可以省略do.

问题描述:

people make a promise will do whatever they can (do).为什么可以省略do.

因为主语的谓语动词是do,这时为了避免重复,其后的从句中情态动词can 后的do 可以省略
另:People 后应该有个who,who make a promise 是定语从句