she tried them on.on试穿鞋为什么用on

fit on
try-on
try on
试穿

try 是动词,此时on用于其后作副词.于是可以看看它作副词的含义：
副词
意思：
1. 穿上,盖上,连接上 It's cold today. Why don't you put your overcoat on? 今天很冷.为什么不把大衣穿上?
2.继续 They talked on until midnight. 他们一直谈到深夜.
3. 向前
4. (机器)处于工作状态;开着;通着 He left the engine on while waiting for Joe. 他在等候乔时让汽车引擎开着.
5. 上演着;在进行中