sad是形容词。

难过的；悲哀的，令人悲痛的；凄惨的，阴郁的（形容颜色）的意思。如：

①She looked sad and tired.她看上去伤心且疲惫。

②I have a sad news for you.我有个令人遗憾的消息告诉你。

③We were all sad at her going.我们对她的离去都很难过。

④We are very sad to hear that you are leaving.听说你要走了，我们十分难过。

⑤Separation from his friends made him sad .与朋友们分离使他伤心。