outdoor 什么时候加s，什么时候不加？

outdoor 形容词，作定语outdoor sports
outdoors 形容词，作表语，He is outdoors.
outdoors 还能作副词 There is intolerable noise outdoors.

outdoor 形容词
1. 室外的,户外的
an outdoor tennis court 室外网球场
2. 在室外使用的;在室外进行的
outdoor clothes 室外服装
Badminton is an outdoor game.羽毛球是一种户外运动。

outdoors. 副词
在室外,在屋外;往室外
It's raining now. You had better not stay outdoors.
现在在下雨，你最好不要呆在外面！

词性不一样
outdoor是形容词
outdoors是副词

当该词词性为副词时要加S，词性为形容词时就不需要加S了。