upset为什么不加s

1. upset 有多种词性
   

   形容词： 表心烦的 +  by/about/at sth或者接从句

   动词：（过去式过去分词均为原型）
   

   MAKE SB UNHAPPY 使某人生气

   to make someone feel unhappy or worried

   使生气；使心烦意乱

   •Don’t do anything that would upset him. 别做惹他生气的事情。

   CHANGE STH 改变某事

   to change a plan or situation in a way that causes problems

   打乱，搅乱〔计划、形势等〕

   •The chemicals upset the balance of the environment. 这些化学品破坏了环境的平衡。

   MAKE STH FALL 打翻某物

   to push something over without intending to

   打翻，弄翻

   •He upset a bowl of soup. 他打翻了一碗汤。

   名词

   [U] worry and unhappiness caused by an unexpected problem

   〔由意外之事引起的〕忧虑；烦闷；苦恼

   •If you are the victim of a burglary, the emotional upset can affect you for a long time. 如果家里被盗，烦恼不安的心情会影响你很长时间。

   [C] 混乱
   

   [ C ] when a person or team defeats an opponent who is considered to be much better than them

   〔体育比赛等中〕意外的结果，爆冷门

   •There was a major upset when the young skater took the gold medal. 那位年轻的滑冰运动员夺得了金牌，爆了个大冷门。

   stomach upset 肠胃不适
   

   an illness that affects the stomach and makes you feel sick

2. upsets 是upset做为可数名词（C）时的复数或者动词单三

   以上来自朗文及柯林斯词典
   

用于一般现在时主语是第三人称单数时，upset 才需要加s。upset 的过去式和过去分词均是upset 。