People are always neglecting something they can do in trying to do something they can't do.

something they can do个人认为在这句话里做插入语吧，因为去掉不影响句意的理解。

People are always neglecting in trying to do something they can't do.

neglect in doing sth.忽略做某事

是something in doing...我觉得they can do是定语，用来形容something

to neglect in doing sth.忽略某事