做名词时加s，做形容词时不加。做名词表示人类，可加s做复数形式。

humans

n.人

human的复数

Animals are able to hear high-pitched sounds that are inaudible to humans.

动物可以听到人类所听不到的高频声音。

It is shocking that humans can behave with such bestiality towards others.

令人震惊的是，人竟然能对同类下此毒手。