Used as a result after a verb to show that the action cannot be completed or that the goal cannot be reached.

to be unacceptable; to be no good; won't work

(bound form) to fail to happen; to not become something; to have no success or achievement

Used at the end of a sentence to add emphasis to a rhetorical question, often used together with 难道 nán​dào or 莫非 mò​fēi to mean "can it be that...?".